Sequences of the principal neutralizing determinant (PND) of the external envelope protein, gp120, from 245 isolates of human immunodeficiency virus type 1 are analyzed. The minimal set of peptides that would elicit antibodies to neutralize a majority of U.S. and European isolates of human immunodeficiency virus type 1 is determined with the assumption that peptides of a given length including the central Gly-Pro-Gly triad are required. In spite of the hypervariability of the PND, 90% of these 245 sequences include peptides from a set of 7 pentapeptides, 13 hexapeptides, or 17 heptapeptides. Tests of these peptide sets on 78 additional PND sequences show that 95% are covered by the 7 pentapeptides, 94% by the 13 hexapeptides, and 86% by the 17 heptapeptides. To anticipate variants not yet observed, single amino acid mutation frequencies from the 245 isolates are used to calculate an expanded set of the 10,000 most probable PND sequences. These sequences cover 86% of the total distribution expected for the central portion of the PND. Peptide lists derived from this expanded set when tested on the 78 additional sequences show that 7 pentapeptides cover 95%, 13 hexapeptides cover 94%, and 17 heptapeptides cover 94%. These results suggest that peptide cocktails of limited size with the potential to cover a large fraction of PND sequence variation may be feasible vaccine candidates.