Ten benign breast tumours from 9 female patients (8 with fibrocystic disease and 1 with fibroadenoma) and 1 male patient (with gynaecomastia) were processed into slices and individually cultured for 2 days in serum-free Medium 199. [3H]-TdR was added to the culture medium to assess DNA synthesis. The addition of human prolactin to the culture medium (500 ng/ml) significantly (0.05 greater than P greater than 0.01) increased DNA synthesis; all 9 biopsy specimens from the 9 female patients responded positively to this hormone. Ovine prolactin (500 ng/ml) and bovine prolactin (500 ng/ml) increased the mean incorporation of [3H]-TdR into extracted DNA and increased the mean number of [3H]-TdR-labelled cells, but this increase did not reach the 5% level of probability. The sole case of male breast dysplasia analysed in this study did not respond to either human, ovine or bovine prolactin. These results provide evidence that human prolactin and, to a lesser degree, ovine and bovine prolactin are direct mitogenic stimulants to the epithelium in human (female) benign breast tumours.