Transforming growth factors (TGF-beta 1 and TGF-beta 2) are polypeptide growth factors with a wide range of effects on the growth and differentiated function of a variety of cell types. Transforming growth factors of the beta class (TGF-beta) are found in large quantities in bone matrix and are synthesized by osteoblasts. For these reasons, it has been suggested that TGF-beta may play a major role in the regulation of bone cell metabolism. We have studied the effects of porcine TGF-beta 1 and the recently described porcine TGF-beta 2 in a mouse clonal, osteoblastlike cell line MC3T3-E1 that has previously been shown to have many characteristics of osteoblasts. In serum-containing medium, TGF-beta 1 inhibited alkaline phosphatase activity. The inhibition of alkaline phosphatase activity persisted for at least 72 h following a brief (24 h) exposure to TGF-beta 1. TGF-beta 1 also caused a marked change in cell morphology. High doses inhibited collagen synthesis; lower concentrations caused a small increase. Under serum-free conditions, TGF-beta 1 had biphasic effects on alkaline phosphatase activity inhibiting at high but stimulating at low concentrations and had only a slight stimulatory effect on collagen synthesis. Under the experimental conditions used, the effects of TGF-beta 1 on alkaline phosphatase activity and collagen synthesis were independent of effects on cell proliferation. In serum-containing medium, TGF-beta 2 inhibited alkaline phosphatase activity, an effect that was independent of changes in cell proliferation and caused shape changes in an identical fashion to that observed with TGF-beta 1.