AbstractBiopsies of human endometrium were studied for the presence of basic fibroblast growth factor (bFGF). An immunoreactive Mr 18 000 bFGF-like molecule was detected at high levels both in ovulatory cycle and postmenopausal endometrium. This molecule was identified as bFGF on the basis of its molecular weight, its affinity for heparin, its capacity to induce plasminogen activator production and cell proliferation in endothelial GM 7373 cells, and its cross-reactivity with various anti-bFGF antibodies. The levels of endometrial bFGF do not change during the menstrual cycle but they increase significantly after menopause, as evaluated both by biological and immunological assays. Lower levels of an acidic FGF-like activity were also evident in ovulatory cycle endometrium but, at variance with bFGF, no significant increase of this activity was observed in postmenopausal endometrium. These data represent the first characterization of a polypeptide growth factor present in human endometrium.