Investigations of the authors' patients with HIV infection revealed marked changes in the C3 and C4 components of complement. Of 61 investigated patients with HIV infection only 23 patients (37.7%) had values of the C3 component at the level recorded in controls. Values of the C4 component comparable with the control group were recorded in 24 patients (39.3%). From figures 1 and 2 it apparent that in the investigated group there is a striking hypercomplementaemia of both components, as compared with hypocomplementaemia.