A 48 year old man with extensive cutaneous and subcutaneous metastatic malignant melanoma responded to treatment by repeated plasma exchanges, partial surgical debulking of tumor masses, and intravenous Corynebacterium parvum. Circulating immune complexes were measured by the serum C1q binding method. Tumor regression coincided with serum C1q binding reduction to normal levels. Clinical relapse was preceeded by recurrent elevation of serum C1q binding activity.