A total of 50 melanoma patients free of distant metastatic disease and 54 healthy controls were analyzed for circulating immune complexes (cIC) and complement split product (C3d), using solid-phase Clq-anti-IgG radioimmunoassay (RIA), Clq-protein A RIA, and anti-C3d anti-IgG RIA for cIC detection. No significant differences in cIC and C3d levels could be demonstrated between the controls and the 31 patients with primary malignant melanoma analyzed before surgery. To evaluate the prognostic value of serial measurements, samples from the 50 patients were taken at regular intervals for 4 to 27 months (median, 20 months). Surgery was the only treatment given. Significant changes in the cIC and C3d levels were defined by reference to the changes that occurred in 23 of the 54 healthy controls observed for a period of 6 to 55 months (median, 23 months). During the period of serial sampling, recurrent disease developed in 8 of the patients. In only 3 of these 8 patients (versus 10 of 42 patients without recurrence) did significant changes occur, and the changes occurred either at the same time or after the clinical diagnosis of recurrence. During the entire clinical observation period of 6 years, a total of 11 patients developed recurrences. Significant changes were only observed in 4 of these 11 patients versus 8 of 37 patients without recurrence. In conclusion, changes in cIC and/or C3d levels were not found to be indicative of early or long-term recurrence of malignant melanoma.