A definitive conclusion regarding the potential for the presence of neutralizing antibody to epitopes of the V3 loop to attenuate or prevent vertical transmission of HIV infection cannot be made based on the results of these four studies. However, the studies provide an intriguing suggestion that it may be possible to identify an epitope or array of epitopes from the V3 loop of judiciously selected HIV isolates that could induce protective immunity against HIV. Future collaborative studies are needed to standardize and independently validate the various assays. A recent conference, Early Diagnosis of HIV Infection in Infants, sponsored by the National Institute of Child Health and Human Development, the National Institute of Allergy and Infectious Diseases and the Centers for Disease Control, provided a forum to facilitate exchange of information among researchers in this field, and encourage future collaborative efforts. If one or more subpopulations of anti-gp120/V3 loop or other neutralizing antibodies are indeed associated with a decreased risk of maternal-infant transmission, it becomes important to determine whether these antibodies are protective per se or are merely surrogate markers of a different mechanism. The former conclusion seems to be supported by the finding of HIV-specific antibodies in seropositive persons' saliva and breast milk, both of which have relatively low infectivity, although the potential neutralizing activities of these antibodies have not been adequately evaluated. The development and testing of a highly specific (monoclonal antibody-derived) passive immunotherapy will probably be required to answer this question.(ABSTRACT TRUNCATED AT 250 WORDS)