Bladder cancer patients from 2 different medical centers were examined to establish whether such patients have circulating immune complexes. Four methods of analysis were used: 1) polyethylene glycol precipitation, 2) double crossed immunoelectrophoresis, 3) Raji cell and 4) Clq binding assays. In the first group of 24 patients 17 had positive results on cystoscopy for pathologically defined tumors at the time of the serum sample. Two tested positive for the presence of circulating immune complexes by all 4 techniques and an additional 1 tested positive by 3 of the 4 techniques. In the second group of 54 patients (41 of whom had pathologically definable tumors at sample date) 9 were judged possibly positive by the Raji cell assay, the polyethylene glycol and double crossed immunoelectrophoresis techniques. When tested by the Clq binding assay 8 of the 9 patients were positive, most being in the range of 260 to 320 microgram/ml. immune complex. Combining all data from the 78 patients with bladder cancer the results in 10 cases definitely were positive by all 4 techniques and an additional 2 were positive by 3 techniques. Our data indicate that a low percentage (13 to 15 per cent) of patients with bladder cancer has circulating immune complexes. Of interest is that the complexes, as judged by our assay procedures, bind Clq, contain aggregated IgG and can be dissociated into antigen and antibody. Thus, the immune complexes are similar to those found in immune complex diseases.