A neural network computer program, trained to predict secondary structure of proteins by exposing it to matching sets of primary and secondary structures from a database, was used to analyze the human immunodeficiency virus (HIV) proteins pl7, gpl20, and gp41 from their amino acid sequences. The results are compared to those obtained by the Chou-Fasman analysis. Two α-helical sequences corresponding to the putative fusigenic domain and to the transmembrane domain of gp41 could be predicted, as well as a possible binding site between pl7 and gp41. On the basis of the secondary structure predictions, a three-dimensional model of pl7 was constructed. This model was found to represent a stable conformation by an analysis using an energy-minimization program. The model predicts that pl7 is attached to the membrane only by the acylated N-terminus, in analogy with the N-terminus of the gag protein of other retroviruses and also with the src oncogene protein p60src. The intracellular C-terminal part of gp41 may act as a receptor by electrostatic interaction with pl7.