Circulating immune complexes were identified in 143 patients with malignant tumors (70--solid cancer, 37--acute leukemia, 21--chronic lympholeukemia and 15--myelomatosis) as well as in 64 patients with chronic inflammatory diseases. Immune complexes were detected in 40% with the aid of precipitation with polyethylene glycol alone and in 50%--by using it in combination with an anticomplementary method. Immune complexes occurred more often in acute myeloid leukemia (66%) than in pancreatic cancer (23%) or chronic lymphoid leukemia (29%). The immunoglobulin profile of complexes varied: in patients with pancreatic cancer, mainly IgM was found; in leukemic patients--different combinations of IgM, IgA and IgG. In the immune complexes of some patients with pancreatic disease, CEA and normal pancreatic antigens were detected.