Hepatocyte growth factor (HGF) is a novel mitogen for mature hepatocytes. In the present study, we have measured immunoreactive (ir)-HGF concentration in tumor extracts of 82 primary human breast cancers using an enzyme-linked immunosorbent assay (ELISA). Ir-HGF was detectable in all tissue extracts, the concentration ranging from 1.4 to 306.5 ng/100 mg protein (median value: 11.2 ng/100 mg protein). Correlation analyses between ir-HGF concentration and clinicopathological factors showed that the ir-HGF level was significantly higher in tumors with sizes of more than 5.0 cm compared with those less than 5.0 cm. In contrast, no detectable amount of ir-HGF was secreted into culture medium of two breast cancer cell lines, MCF-7 and ZR-75-1, suggesting that the cancer cell itself has no ability to produce ir-HGF.