We have previously reported the detection of anti-CD4 autoantibodies in the sera of approximately 10% of HIV-1-infected patients. These antibodies recognized epitopes located on the V3V4 domains of the soluble recombinant CD4 molecule but did not recognize CD4 in its native conformation on the cell membrane. To analyse further the biological properties of these antibodies, we have purified them from the sera of six patients using affinity chromatography. We demonstrate that purified and concentrated anti-CD4 antibodies still cannot recognize CD4 either in its native conformation or after the binding of HIV gp120. They also cannot mediate antibody-dependent cellular cytotoxicity on CD4+ or HIV-coated CD4+ cells. In one patient, however, we could detect a minor fraction of anti-CD4 antibodies directed to the V1V2 domains of CD4.