Production of escape mutant viruses may actually occur during the normal course of HIV infection and therefore escape being neutralized by the immune system. In our study, escape mutant viruses have been selected by a limiting dilution method in which we have chosen those that have been able to grow in the presence of a dilution of human serum which is neutralizing under normal circumstances. After a few selection experiments, we have been able to identify four different escape mutants of the MN strain of HIV. These mutants have been sequenced and do show nucleic acid and/or amino acid differences within and also outside of the V3 loop, which is the third variable domain of the gp120 and is also known as the principal neutralizing domain or PND. The mutants are presently being run by Polymerase Chain Reaction (PCR) again and will be sequenced once again to determine if the previous differences will again be detected. Further characterization of these mutants will be done in the future. The significance of this study involving escape mutants may be relevant for future vaccine work. PHS 6040 (Rev. 5/92)