C and CR1 have been shown to participate in the clearance of injected, preformed, immune complexes in humans and in non-human primates. Their role in the physiologic disposal of immune complexes formed in vivo in humans was investigated in three patients receiving radioimmunotherapy for ovarian carcinoma. On day 0 each patient received, by intraperitoneal injection, 10 mg of 131I-mouse anti-tumor mAb (10 mCi/mg). On days 1 and 2, 18 mg of trace-labeled, 125I-human anti-mouse IgG was administered by i.v. infusion over 15 min, to accelerate the clearance of the 131I-anti-tumor antibody from the circulation and reduce the radiation dose to the marrow. Sequential blood samples were obtained after the injection of the second (anti-mouse) antibody, to monitor clearance. Immune complexes (shown by sucrose gradient centrifugation to be 19 to 40 S in size) formed within 5 min, and were cleared with a half-life of 11 +/- 1.7 min in the liver. Complexes were measured by 4% polyethylene glycol precipitation, and by solid phase C3d- and C1q-binding assays. Between 8 and 11% of the total available complexed material bound to CR1 on E. Peak binding of immune complexes to red cells occurred 10 min after the maximal complex load was detected by precipitation with polyethylene glycol. At that time, immune complexes bound to E constituted one-fifth of the total circulating pool of complexes. Coincident with immune complex formation and clearance, a 47% fall in serum C4, C3, and CH50 was measured, with the deposition of up to 1230 molecules of C4, and 2590 molecules of C3 on the surface of red cells. During 20 min after immune complex formation there was a mean loss of 32% of erythrocyte CR1. The changes in complement and CR1 on E and in serum observed in these patients resembled those seen in patients with SLE: i.e., a reduction in CR1 and an increase in C3 and C4 on E, and reduced serum C.