Expression of the retinoic acid receptors a and (3 (RAR-a and RAR-(3) was examined in F9 cells, an embryonal carcinoma cell model established for the study of retinoid metabolism and function. Addition of retinoic acid to F9 cell medium caused a dose-dependent increase in RAR-fi mRNA within 3 hr that reached 5-to 30-fold greater than the constitutively expressed mRNA by 24 hr. ized by loss of tumorigenicity, decreased proliferation, and large changes in the expression of plasminogen activator, laminin, collagen IV, and c-myc product, as well as in the cell surface antigen SSEA-1 (6, 9-11). The most potent of the naturally occurring retinoids in promoting F9 cell differentiation is retinoic acid (6,9,12,13). Retinoic acid is a physiologically occurring and activated metabolite of retinol (14) that appears to act directly in stimulating F9 cell differentiation (12). The discovery of three distinct retinoic acid receptors, a,