In the Y1 mouse adrenal tumor cell line, the 3β-hydroxysteroid dehydrogenase isomerase enzyme (3β-HSD) which catalyzes the transformation of 3β-hydroxy-5-ene steroids to 3-keto-4-ene steroids is active.The effect of typeβ1 transforming growth factor (TGFβ1), a potent modulator of adrenocortical differentiated functions, on the 3β-HSD enzyme was studied. Four isoforms of 3β-HSD yielding proteins of different mobility on SDS-PAGE were previously detected in the mouse; whereas only one form was present in the mouse adrenal, we detected two isoforms in the Y1 cells. An inhibition of the basal enzymatic activity was observed after TGFβ1 treatment which was correlated with a decrease in 3β-HSD protein (both isoforms) and m-RNA levels.