Three complement-dependent and one rheumatoid factor-dependent immune complex assays were used to analyze the sera taken before surgery from 70 unselected and previously untreated lung cancer patients, from 30 of them after surgery, and from 31 healthy controls. Plasma levels of complement split product C3d were also analyzed. The levels of circulating immune complexes (cIC) and C3d were essentially the same in samples taken from lung cancer patients before surgery and from healthy controls. By the four immune complex assays, increased levels were found in 0 to 10% of the preoperative lung cancer patients compared to 3% of the healthy controls (not significant). The postsurgical tumor, lymph node, metastasis (pTNM) stage of the lung cancer was not reflected in the levels of cIC or C3d. Paired comparisons of the cIC and C3d levels before and after surgery did not show significant differences. Thus, we found no evidence for the occurrence of cancer-related cIC in lung cancer patients.