Human Sertoli cells were grown in a serum-free environment, and the Sertoli cell conditioned medium (hSCCM) was tested for mitogenic activity. The presence of a potent growth factor(s), termed Sertoli cell secreted growth factor (SCSGF), in hSCCM was confirmed and supports previous observations based on experiments using rat SCCM. Mitogenicity of hSCSGF was demonstrated in cell proliferation assays with the A431 (human epidermoid carcinoma) cell line and in [methyl-3H]-thymidine incorporation (DNA synthesis) assays with the Swiss 3T3 (mouse embryo fibroblast) cell line. In a dose-dependent manner, hSCSGF stimulated A431 cell growth up to 4-fold over control values (P less than 0.0001) and stimulated thymidine incorporation up to 4.5-fold over control values (P less than 0.0002). Importantly, SCSGF stimulated A431 proliferation 2-fold over control values (P less than 0.0002) in the presence of 5% serum. With the exception of rat SCSGF, human SCSGF is the only growth factor known to stimulate A431 cells. SCSGF also demonstrated epidermal growth factor (EGF)-like activity based upon displacement of EGF from its receptor in a radioreceptor assay. However, SCSGF is not EGF since it is a potent stimulator of A431 cells, whereas EGF is inhibitory. The growth factor was stable to heat, freeze-thaw, acid (pH 3), and trypsin treatment. Furthermore, it did not bind heparin agarose and is thus distinct from the endothelial cell growth factor family. High-pressure liquid chromatography on size exclusion (TSK G2000 SW) columns revealed an approximate size of 8000 daltons. Human SCSGF is a unique growth factor and may play a key role in the regulation of normal spermatogenesis.