5-min boiling of 3-fold diluted serum leads to the quantitative release of immune-complexed HIV antigen (Ag) that can be readily measured. The method is capable of detecting as little as 4 to 5 pg/ml Ag p24 depending on the Ag test used. Of a panel of 50 anti-HIV-positive sera, 30 (60%) were Ag-positive after heat denaturation, 17 (34%; P < or = 0.01) after acidification, and 13 (26%; P < or = 0.001) when tested undenatured. The superiority of heat denaturation was greatest in sera with high-titered antibodies to p24. Occurrence and mean concentrations of heat-denatured Ag were reversely correlated with p24-specific antibody. It is suggested that heat-denatured proteins be used for raising antibodies that will serve as capture or trace antibodies in Ag assays. This simple change in the manufacturing of test reagents in combination with a short boil before testing may solve the problem of incomplete Ag detection due to immune complex formation.