High concentrations of fetal bovine serum induced colony formation in soft agar by anchorage-dependent, nontransformed mouse AKR-2B and rat NRK cells. The colony-stimulating activity in fetal bovine serum was precipitated by 45% saturated ammonium sulfate and migrated in molecular sieve chromatography as a single peak of activity in the 10,000-15,000 molecular weight range. The colony-stimulating activity was heat and acid stable and was destroyed by trypsin and dithiothreitol, indicating the activity is due to a polypeptide that requires disulfide bonds for biological activity. No competition for binding to the epidermal growth factor receptor was associated with the colony-stimulating activity. Isoelectric focusing revealed activity in the pI 4-5 range. The colony-stimulating activity in serum appeared to be of platelet origin because platelet-poor plasma and platelet-poor plasma-derived serum contained little activity, whereas acid/ethanol extracts of bovine and human platelets had potent colony-stimulating activity. Chromatography of platelet extracts on Bio-Gel P-60 revealed peaks of AKR-2B colony-stimulating activity in the 12,000 and 20,000 molecular weight ranges. The other biological and chemical properties of the platelet colony-stimulating activity were the same as those for the serum activity. The data indicate the presence in serum of a platelet-derived growth factor(s) with properties similar to those of the transforming growth factors.