A multicenter study of CA549, a marker for breast cancer, was conducted using sera from 1721 patients with benign and malignant conditions by an immunoradiometric assay, BRESMARQ. Acceptable assay performance was demonstrated by studies of intra-assay (2.2% to 12% coefficient of variation [CV]), interassay (4.1% to 11.8% CV), and interlaboratory (4.8% to 8.7% CV) precision; sensitivity (.3 kU/L); linearity; recovery; high-dose hook effect (up to 10,000 kU/L); and interferences (human antimouse antibodies; protein, bilirubin, hemoglobin levels; lipid and cancer therapeutic agents). A reference interval of 0-12.5 kU/L (women) and 0-11.9 kU/L (men) was established from 746 healthy persons. The distribution of values exceeding the reference range for benign conditions was as follows: pregnant and lactating women (2%); benign breast disease (5%); and seven other benign diseases, including liver (24%), lung (19%), prostate (14%), colon, endometrial, gastric, and ovarian (< 10%). For nonbreast cancers, the distribution was Hodgkin's (7%), colon (10%), endometrial (15%), gastric (15%), lymphoma (15%), prostate (20%), ovarian (39%), and liver (45%). For breast cancer, the distribution was stage I (5%), stage II (14%), stage III (32%), and stage IV (74%). The receiver-operating characteristic analysis demonstrated the usefulness of CA549 as a marker in stage IV breast cancer.