Circulating immune complexes (CICs) containing polymorphic epithelial mucin (PEM/MUC-1) were found in sera of 24.5% of 151 primary breast carcinoma patients and 18-21.4% of patients with advanced ovarian (n = 56) and breast carcinomas (n = 61), 37% of patients with benign breast tumours, but in only 2.1% of 96 healthy individuals. The incorporation of PEM into CICs affects the detection of circulating PEM in commercial immunoassays such as the CA 15-3 assay, as suggested by a negative correlation between levels of PEM-containing immune complexes (PEM-CICs) and CA 15-3 values, and confirmed by isolation of PEM from CA 15-3-negative sera containing high levels of PEM-CICs. The amounts of PEM masked by human antibodies correspond to significant values of the CA 15-3 assay when monitoring patients for carcinoma. Most antibodies in PEM-CICs were of IgG class, suggesting their specific nature to the PEM epitopes.