In addition to MEDLINE's bibliographic information, however, PubMed also has citations, abstracts, and links contributed by participating publishers of biomedical literature. Publisher-supplied links from PubMed citations take users to the e-journals stored at the publishers' Web sites, where access may be dependent upon subscription or other fees. Linking in PubMed falls under the jurisdiction of LinkOut (http://www.ncbi.nlm.nih.gov/linkout), a feature that allows information providers to supply links to their Web-accessible resources from citations or other data in PubMed and additional Entrez databases (http://www.ncbi.nih.gov/Entrez).

EBSCO Publishing's announcement brings to our attention the fact that publishers are not the only game in LinkOut. Whereas LinkOut initially began with links to e-journals, other information providers have also begun taking advantage of this powerful, free tool. According to Kathy Kwan, coordinator of the LinkOut project, anybody can participate in it.

Anyone can participate, and many do. In this regard, EBSCO Publishing is not unique, as many other information providers, including other aggregators and libraries, also make their full-text materials available via LinkOut. The catch, of course, is that such full text is only accessible for affiliated users. There is the possibility of free full text through links to such resources as BioMed Central (http://www.biomedcentral.com) or PubMed Central (http://www.pubmedcentral.nih.gov), but full text for free is certainly the exception.

EBSCO Publishing actually began hooking libraries' EBSCO Publishing content up to NLM's PubMed approximately 60 days before the February 14 announcement. According to Robert Preston, EBSCO Publishing's sales manager for special libraries, "NLM was cautious about this whole thing, but it turned out to be a painless experience."

When asked why EBSCO Publishing decided to become a LinkOut provider, Preston said that it just made sense for his company. "We figure it's just another value-added service. It's a quid pro quo relationship. We're actually providing a service that allows NLM to market itself."

EBSCO Publishing seems to be one of the first big aggregators to climb on board, but it may not be the last. Kwan noted: "We welcome other aggregators. It's a voluntary program, and we hope people know about it." Kwan went on to say that libraries have expressed interest in the possible participation of aggregators such as ProQuest (http://www.proquest.com). Time will tell who chooses to join the ever-expanding list of LinkOut providers.

This number of LinkOut providers continues to grow (http://www.ncbi.nlm.nih.gov/entrez/journals/active_providers.html). Three new providers were added as recently as late February, bringing the total to 455, a number that includes publishers, aggregators, libraries, and other information providers.

The benefits of LinkOut are clear. An affiliated user in a library or a company can go directly from a citation and abstract to the full text of the cited article if his or her library or company has a subscription to either the e-journal or the aggregator service. Kwan emphasized that all contributed links must be to the article level, not to the information provider's front door. And with the customization features offered through LinkOut for Libraries (http://nnlm.gov/libinfo/ejournals/linkout), libraries can include icons to point users to the appropriate copies of any given full-text materials. With the Library Files Submission Utility (http://www.ncbi.nlm.nih.gov/entrez/linkout/doc/lsub-i.html), libraries can mark holdings for their users.

EBSCO Publishing provides some administrative support for libraries that want their EBSCO Publishing holdings to appear in PubMed. Through LinkOut for Libraries, these libraries can register for an account. After registering, they can then input holdings information (EBSCO and otherwise) through the LinkOut Holdings Submission Utility. It's important to note, however, that only those holdings corresponding to participating LinkOut providers may be entered. This process is entirely free.

Additional personalized customization is available via LinkOut's Cubby service (http://www.ncbi.nlm.nih.gov/entrez/login.fcgi?call=so.SignOn..Login), a profiling tool for users. Any user can register for a Cubby account, with which he or she can save searches, select and deselect LinkOut providers, and/or change document delivery services. The PubMed default document delivery service is Loansome Doc, but other available services include Infotrieve, TDI Library Services, and TheScientificWorld, Inc.

With Cubby, users can set up their PubMed interface so that only certain provider icons appear. For example, a user affiliated with a certain library (that participates in LinkOut) can choose that library's icon. Cubby also provides the option of viewing all the new LinkOut providers since the last login. Both the Cubby and LinkOut for Libraries features provide the sort of localized customization that offers seamless linking for affiliated users.

Of course, there are a couple of snags in this seamless linking environment. First, if a library has more than one version of any given Web-accessible resource, such as a journal article, the library must choose its preference or LinkOut will default to the publisher's version.

Second, and more importantly, for the unaffiliated user, seamless or free access to resources via LinkOut may be severely limited. Document delivery and pay-per-view are options, and any full text with restriction (fee or subscription access) is marked by a "*" sign on the LinkOut display of a citation. Additionally, no list currently exists of providers that offer pay-per-view or any free full text. When asked about plans to include such lists, Kwan said, "We will keep it as a suggestion for future development."

There's no doubt about it: Linking is hot. And for the affiliated user with access via institutional or personal subscriptions, access to full text can be seamless. For the unaffiliated, however, access is less plentiful.