The presence of immune complexes and carcinoembryonic antigen (CEA) was investigated in 50 patients with bronchogenic carcinoma at the time of and/or following diagnostic or definitive surgery. Immune complexes were measured by the C1q binding test and CEA by the Z gel method and elevations defined as values in excess of 2 S.D. above the normal mean, greater than or equal to 9.2% for C1q binding activity (C1q-BA) and greater than or equal to 5.0 ng/ml for CEA. The overall incidence of elevated values was 30.7% for C1q-BA and 34.2% for CEA. There was a greater incidence of elevated values of C1q-BA among patients with clinically evident disease. The differences with respect to CEA elevation were not significant due to the fact that 6 of 9 samples with elevated CEA values obtained from patients with no evident disease were in fact associated with the presence of clinically undetectable disease in these patients. Elevation of C1q-BA and CEA beyond the immediate postoperative period was predictive of a significantly shorter median survival time. The most significant differences in survival time were seen between patients with normal values for C1q-BA and CEA and those with elevations of one or both parameters, 6.0 vs. 19.5 months (p less than 0.001). Elevation of either parameter during the immediate pre- and postoperative period was not predictive of a poor survival. In terms of clinical application, it appears that CEA estimation had the best predictive value but that the addition of C1q-BA measurement may provide additional prognostic information, particularly in patients who do not have elevated CEA values.