We have studied the expression of the C3b receptor (CR1) on erythrocytes of 55 patients with Hansen's disease. We developed a radioimmunoassay utilizing a monoclonal antibody that recognized an epitope different from the C3b binding site, which therefore enabled us to measure total number of CR1 regardless of receptor occupancy. We observed that patients in the lepromatous pole of the disease had a mean of 310 CR1/erythrocyte, whereas the ones in the tuberculoid pole showed a mean of 577 CR1/erythrocyte; 77 normal controls had a mean of 512 CR1/erythrocyte. The number of C3b receptors on the cells of lepromatous patients was significantly decreased (p < .001) when compared to the normal population of tuberculoid patients. The presence of receptors for the C3b fragment of complement (CR1) on the surface of human erythrocytes enables these cells to participate in a number of immune functions including the clearance of circulating immune complexes. These findings could bear importance in the ability of the host to clear immune complexes from the circulation in patients with lepromatous leprosy.