The effect of beta-retinoic acid (RA) on carcinogen-induced hyperplasia was studied in organ cultures of mouse prostate gland. 3-Methylcholanthrene (MCA), requiring metabolic activation, or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), not requiring activation, were used to induce hyperplastic changes. Treatment of cultures with MCA or MNNG stimulated cell proliferation and caused the alveolar epithelium to become hyperplastic. The development of this hyperplasia was inhibited when RA was added simultaneously with MCA or MNNG. However, RA had no significant effect on cell proliferation in untreated control cultures. Elimination of carcinogen from the hyperplastic cultures after 8 days of treatment did not reverse hyperplasia of the alveolar epithelium. When the withdrawal of MCA or MNNG was followed by treatment of the cultures with RA, hyperplasia was markedly reversed within 96 hours. Thus RA actively inhibited and reversed the effect of MCA and MNNG, two carcinogens that may have different mechanisms of action.