We investigated the antigenicity of a highly conserved region in the transmembrane protein of the human immunodeficiency virus type 1 (HIV-1). In order to identify antigenically important residues, amino-acid sequences of synthetic peptides representing this region were varied systematically: single residues were omitted from the sequence of HIV-env 583-599; threonines were substituted for pairs of residues in HIV-env 581-599; the sequences of heptadeca-peptides were shifted by single residues. The peptides were tested in an enzyme immuno-assay against fourteen HIV-1 antibody-positive human sera, which were previously found to react with HIV-env 583-599, and against rabbit antisera to the peptides HIV-env 583-599 and 586-606. Substitutions as well as deletions in the sequence 589-596 (AVERYLKD) aborgated the antigenicity of the peptides with most of the human sera. Changes outside this sequence affected the reactivities differentially. Six overlapping dodeca-peptides, shifted in the sequence by single residues, lacked antigenicity in a competition assay, suggesting antigenic dependence on an ordered peptide conformation, which the longer peptides may preferentially assume. 19- and 21-mers with overlapping sequences competed to different extents with each other for binding to the antibodies of 3 human sera, illustrating that more than one antigenic structure in this narrow region can be recognized by a single polyclonal serum.