In this study, we report that retinoic acid enhances dramatically the colony-forming ability in soft-agar medium of normal rat kidney fibroblast cells NRK 536–3 (SA6) exposed to sarcoma growth factor. Retinoic acid also stimulates colony formation in soft agar induced by the phorbol ester 12-O-tetra-decanoylphorbol-13-acetate, although the phorbol ester alone is less effective in producing anchorage-independent growth than is the sarcoma growth factor. Retinoic acid not only enhances the number of colonies formed but also causes an increase in their average size. Retinoic acid by itself does not induce anchorage-independent growth. The formation of colonies in agar induced by either the sarcoma growth factor or the phorbol ester is a reversible characteristic whether retinoic acid is present or not. Retinoic acid acts synergistically with the sarcoma growth factor and the phorbol ester on anchorage-dependent growth of these NRK cells. In addition to retinoic acid, the 13-cis-, 4-oxo-, and trimethylmethoxyphenyl analogues of retinoic acid and retinol also stimulate colony formation in agar induced by sarcoma growth factor or the phorbol ester. Although the stimulation of growth in soft agar by various retinoids correlates well with the specificity with which they bind to the cytosolic binding proteins in other systems, no binding proteins could be detected in the cytosol of these normal rat kidney cells. © 1983, American Association for Cancer Research. All rights reserved.