Anti-recombinant human basic fibroblast growth factor (rbFGF) antibodies efficiently inhibited the basal proliferation of bovine pulmonary artery endothelial (BAE) cells. The cell-free extract of BAE cells stimulated the proliferation of bovine capillary endothelial cells and this activity was completely abolished by the antibodies. Furthermore, on heparin HPLC the activity was eluted at exactly the same retention time as that for authentic pituitary bFGF. These observations directly indicate that the BAE cells produce bFGF that stimulates their own basal growth by binding to specific receptors expressed on the cell surface.