Retinoic acid, the acid form of vitamin A, was found to restore anchorage-dependent growth to three transformed cell lines in vitro but not to four others. The ability of retinoic acid to restore this characteristic of normal cell growth was found to vary across a wide spectrum among the different cell lines tested. L-929 cells were the most sensitive; in this cell line 2.1 × 10-9 M retinoic acid was capable of inhibiting clone formation by 50% in methyl cellulose-containing medium. Retinoic acid induced anchorage-dependent growth of L-929 and HeLa cells was found to be reversible. Other vitamin A analogues were used in the clone inhibition assay and it was found that the effect was specific for retinoic acid. Since anchorage-independent growth may be related to tumorigenicity, this effect of retinoic acid on transformed cells may be the basis of the anti-tumor properties of this vitamin. © 1978.