Sera from 134 selected patients with various types of cancer were tested for soluble antigen-antibody complexes by the C1q binding method. Sera from 85 healthy blood bank donors served as normal controls. C1q binding activity (C1q BA) values above the 95th percentile for healthy subjects were found in 83% of sera from patients with neoplastic diseases. The incidence of abnormal C1q BA values among patients with malignant melanoma was 83%, with breast cancer 74%, with colon cancer 75%, with lung cancer 88%, with leukemia and lymphoma 85%, and with miscellaneous tumors 94%. High C1q BA values were found most frequently in sera of patients who had been diagnosed relatively recently (within 5 mo) and who had evident residual disease after surgical treatment. Recurrence or progression of tumor growth occurred significantly more frequently in lung cancer patients with high C1q BA. DNA was not detected in cancer patients' sera and treatment with DNase did not decrease in C1q BA. C1q BA in sera could not be explained by the presence of antiglobulin antibodies. Sucrose density gradient ultracentrifugation studies of the serum C1q BA in 4 cancer patients showed that the major binding activity was found between 19S and 7S.