Neutralizing and complement-mediated infection-enhancing antibodies to HIV-1 were measured in sera or plasma from 54 HIV-1-positive individuals at various stages of disease, and from an additional 36 HIV-1-positive individuals for whom no clinical data were available. Antibodies were measured in microtiter infection assays utilizing MT-2 cells and the IIIB strain of HIV-1. The frequency of detection of both types of antibodies was identical, being 77 out of 90 cases (86%). Neutralizing and infection-enhancing antibodies were not always found together, and in four cases both were undetectable. No correlation was found between liters of either type of antibody and stage of disease. Furthermore, liters of infection-enhancing antibodies at early stages of disease did not predict rate of disease progression.