Circulating immune complexes (CIC) in human cancer are known to be very heterogeneous in size and composition. In 95 staged malignant melanoma patients and 71 individuals with leukemia and lymphoma, this heterogeneity was analyzed biochemically in sera positive for CIC. CICs were measured by a multiassay system and individual complexes were isolated and analyzed by immunological and biochemical methods. Analyses of sera from 100 normal individuals, from 25 rheumatoid women, and a group of 12 laboratory staff who work with human melanoma were included for comparison. Three basic patterns of complexes were identified circulating in the sera of the cancer patients. Type I are medium-sized (17-23S), complement-fixing complexes usually occurring in combinations. The prototype in melanoma contained IgG antibody and additional glycoprotein components and bound complement by the classical pathway. In hematological malignancies four subtypes could be identified depending on whether the antibody class was IgG or IgM, the nonimmunoglobulin component was glycoprotein or protein, and whether complement fixation occurred by the classical or alternate pathway. Type II complexes were noncomplement-fixing, medium-sized complexes (15-21S), which in melanoma contained IgG antibody and additional protein components. In the hematologic malignancies two subtypes could be identified depending on whether the antibody class was IgG or IgM. Both subtypes contained a glycoprotein nonimmunoglobulin component. Both melanoma and hematologic tumors had type III heavy complexes (36-44S) which were noncomplement-fixing and contained only immunoglobulin components, either IgG-IgG or IgM-IgG. As expected the rheumatoid arthritis patients frequently had both 7S and 21-23S CICs containing IgG as well as IgM rheumatoid factor with complement fixation via the classical pathway. No CICs were detected in normal young men and women (20-30 years); a few individuals in middle age (31-50 years) had small (7-11S) CICs which bound complement by the classical pathway and contained IgG and a protein nonimmunoglobulin component. The frequency of these 7S complexes increased with advancing age, with the appearance of 23S IgG-IgG or IgM-IgG complexes. IgG antibodies from only the melanoma patients reacted with cytoplasmic components of fresh melanoma cells, except the laboratory workers where all of their isolated CIC antibodies also reacted with melanoma cells. Thus the heterogeneity of complexes in melanoma is not random, but can be classified into three basic biochemical patterns. The hematologic group provides a slightly richer variation of subtypes within this basic scheme.