Apolipoprotein H (apo H), isolated from human plasma albumin solution, was shown to capture HIV-1-related antigens from antigen-positive sera (HIV-1 AG+) of AIDS patients, by using HIV-1-specific polyclonal antibodies. In an enzyme-linked immunosorbent assay and ligand blot and dot assays, apo H was able to bind recombinant retroviral HIV antigens, especially Gag proteins p18 of HIV-1, p26 of HIV-2, and Env gp160 of HIV-1. Binding was shown to be pH and NaCl dependent, with an optimum at acidic pH and low ionic strength. Specificity was demonstrated by saturation of this binding and inhibition either by homologous competition or by specific antisera. Binding was also observed in cell line-harvested viral proteins. The mechanism of this apo H-polyspecific binding is discussed in relation to conformational changes due to the influence of lipids or detergents.