A total of 18 overlapping peptides were synthesized that covered envelope amino acid sequences (amino acids 288-472 of the III-B isolate) of the human immunodeficiency virus type 1 (HIV-1). Antibodies from human immunodeficiency virus-1-infected individuals bound to three of the peptides tested. Guinea pigs were immunized with each of the overlapping peptides and the resultant sera analyzed for biologic activity. One of the peptides elicited antibodies that had both neutralizing and fusion blocking activities that were type specific. This peptide, designated 1-69, was also one of the peptides reactive with the human immunodeficiency virus type 1-positive human sera.