Two groups of sera were investigated with two different enzyme immunoassays for the detection of anti-LAV/HTLV-III-antibodies: 50 sera collected from patients with lupus erythematosus and 5 sera containing anti-HLA-DR4 antibodies. Of the 50 sera of the lupus erythematosus patients, 2 were positive in each of the tests. Of the 5 sera that contained anti-HLA-DR4 antibodies, one was positive. In addition, 11 sera showed positive results after heat treatment (56 degrees C, 1 h) in one of the tests. However, all sera in this group gave negative results in the other test. The positive results could not be confirmed with the immunoblot. These findings show that one must be aware of false-positive results when the sera of certain types of patients are investigated in anti-LAV/HTLV-III enzyme immunoassays. The sera of patients with lupus erythematosus, as well as that of polytransfused persons or multiparous women, that have been sensitized with HLA-DR4 antigens; heat treatment in one test has an effect on sera.