The effects of porcine bone morphogenetic protein (BMP) on DNA and collagen synthesis in cultured rat osteogenic sarcoma cells and mink lung epithelial cells were studied and compared with the effects induced by transforming growth factor-beta 1 (TGF-beta 1). In both cells, DNA synthesis was slightly but significantly increased by BMP whereas it was notably decreased by TGF-beta 1. The inhibitory action of TGF-beta 1 overrode the activation of DNA synthesis by BMP when the cells were incubated together with TGF-beta 1 and BMP. In osteogenic sarcoma cells, collagen synthesis was enhanced by both BMP and TGF-beta 1, but the stimulatory action of BMP was weaker than that of TGF-beta 1. In epithelial cells, TGF-beta 1 increased collagen synthesis but BMP induced no significant changes. No synergistic effects of TGF-beta 1 and BMP on collagen synthesis were observed in both cells. The present study demonstrates the possibility of direct actions of BMP and TGF-beta 1 on cultured rat osteogenic sarcoma cells and mink lung epithelial cells in vitro.