Induction of the transglutaminase activity of a transformed mouse epidermal cell line (PAM 212 cells) by 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2-D3), the active form of vitamin D3, was investigated. Addition of 1 alpha,25-(OH)2-D3 to a culture medium stimulated twice the transglutaminase activity at a concentration of 10(-7) M, but vitamin D3, prostaglandin E1, E2, and F2 alpha failed to show this induction. Phorbol 12-o-tetradecanoylphorbol-13-acetate (TPA) and dexamethasone also induced an increase in transglutaminase activity. Exposure to both 1 alpha,25-(OH)2-D3 and retinoic acid caused remarkably synergistic effects on the induction of transglutaminase in the PAM 212 cells. In contrast, simultaneous addition of 1 alpha,25-(OH)2-D3 and TPA was antagonistic and resulted in less than additive induction. Vitamin D3 also showed a similar but lesser effect. These results suggest that 1 alpha,25-(OH)2-D3 induces the transglutaminase activity via mechanisms disparate from those of retinoic acid and modifies epidermal differentiation.