Anchorage-dependent and -independent MCF-7 cell growth was dose-dependently inhibited by retinoic acid (RA) but was insensitive to TGF-β(from 1 to 100 pm). Growth of MCF-7 monolayer cultures was inhibited (50%) when exposed to 10-6M RA. RA was unable to completely inhibit MCF-7 cell proliferation, as concentrations above 10-6M were rapidly cytotoxic. However, the combination of TGF-βand RA resulted an increase in RA inhibitory effect on MCF-7 monolayer growth and a 80% reduction in colony formation in soft agar. These results demonstrate that although TGF-βdoes not inhibit the growth of MCF-7 cells, it potentiates the antiproliferative effect of RA, suggesting that it may play a part, albeit indirect, in the regulation of MCF-7 cell growth.