An immunocytochemical method involving the application of polyclonal antisera to human prolactin (PRL) followed by a highly sensitive and a modified version of dinitrophenyl (DNP) hapten sandwich staining procedure using anti-DNP IgM monoclonal antibody has been used to detect PRL binding in benign and malignant breast tissue. The technique was applied to 5 microns thick sections of paraffin embedded formalin fixed tissue. Out of 107 breast biopsies 40 were carcinomas, 41 were fibroadenomas, 18 were benign cystic disease and 8 were gynaecomastia. In cases of carcinoma positive staining was observed in 82.5% cases whereas in fibroadenoma the positivity was in 57% cases only. The positive reaction in fibroadenoma was mainly due to the presence of apocrine metaplasia associated with the tumour. Also PRL was present in greater proportion in post menopausal patients as compared to premenopausal cancer patients. These findings suggest the presence of specific PRL binding sites in breast tissue. The staining was restricted to epithelial cells and background staining of the stroma was minimally seen in these cases. Positively stained breast carcinoma may represent an apocrine subset of the carcinoma.