A point mutation (Ala-589 to Thr) in the transmembrane protein of the human immunodeficiency virus type 1 (HIV-1) has been shown to decrease the sensitivity of the virus to the neutralizing effect of human HIV-1 specific antibodies [(1990) J. Virol. 64, 3240-3248]. Here 17-residue peptides with the parental and mutant sequences were compared: the parental peptide bound antibodies of sera from HIV-1 infected persons more frequently and with higher affinity than the mutant peptide. However, according to circular dichroism (CD), NMR spectroscopy and molecular modelling the peptides have indistinguishable backbone conformations under a variety of experimental conditions. These techniques showed for both peptides that no ordered helix was present in water solution. However, for both peptides in alcohol-water solutions approximately 60% alpha-helix could be induced. The three-dimensional structures of these peptides provide a basis for understanding how this mutation in the transmembrane protein may affect the interaction with both the outer envelope glycoprotein and with antibodies.