Breast cancer specimens from 600 women were assayed for estrogen receptors (ER) using an immunocytochemical assay (ICA) employing the monoclonal antiestrophilin antibody H222 Sp gamma. Results showed significant correlation with biochemical ER determinations as well as with tumor grade and menopausal status. In 449 cases, results of progesterone receptor assay by ICA using the monoclonal anti-PgR antibody KD 68, also correlated significantly with biochemical PgR measurements. The ERICA/PgRICA positivity was significantly more frequent in postmenopausal white women. Colloid carcinomas were most likely to be ERICA positive and PgRICA positive whereas medullary carcinomas were most often negative. In 47 patients with advanced mammary carcinoma, results of ERICA and PgRICA were more closely related to endocrine response than those of ER and PgR by dextran-coated charcoal assay (DCC). In 339 women with Stage I or Stage II breast cancer, ERICA was significantly associated with disease-free survival. Analysis by Cox's proportional hazard model, however, showed PgRICA to be the best predictor of survival and disease-free survival in 197 women at the same stages of disease. These data indicate that ICA is more predictive of prognosis than biochemical ER and PgR. The ease of ICA performance coupled with these results indicate that the method is an acceptable substitute for DCC in analyzing breast cancers for ER/PgR.