Immunoreactivity related to the gastrin-releasing peptide (GRP) precursor was detected in four different human breast cancer cell lines. The amounts and the characteristics in extracts from different breast carcinoma cells were compared with cell extracts from small cell lung cancer (SCLC) cells. Two different radioimmunoassays were employed, directed against the amino acid sequence 14-27 of GRP (IR-GRP) or the 42-53 amino acid sequence at the C-terminal end of the GRP precursor (GRP precursor fragment). In extracts from T47D cells cultured under serum free conditions, IR-GRP coeluted with GRP(14-27) or GRP(18-27) in Sephadex G-50 chromatography. No immunoreactivity was detected in the fractions containing high molecular weight components. In a total of 41 human breast carcinoma biopsies from different postmenopausal patients, IR-GRP was detected by immunohistological staining in 39% of the samples. When the GRP(14-27) peptide was added exogenously to breast cancer and SCLC cell lines under serum-free culture conditions, (3H)-thymidine incorporation was stimulated by GRP(14-27) in the SCLC cell lines. Of the breast cancer cell lines only the T47D cell line responded with an increase in (3H)-thymidine incorporation comparable to the increase observed with SCLC cells. Recently, it has been reported that GRP-like receptors are present in some human breast cancer cell lines, including the T47D cell line studied here. The breast cancer cell line T47D therefore expresses the GRP peptide and the receptor for GRP. The identification of GRP-like receptors on T47D cells is in accordance with our present observation of a growth response to GRP(14-27) as evaluated by increased (3H)-thymidine incorporation.