AbstractCyclin‐dependent kinases (cdks) are a family of proteins whose function plays a critical role in cell cycle traverse. Transforming growth factor‐β1 (TGF‐β1) is a potent growth inhibitor of epithelial cells. Since cdks have been suggested as possible biochemical markers for TGF‐β growth inhibition, we investigated the effect of TGF‐β1 on cdc2 and cdk2 in a normal mouse mammary epithelial cell line (MME) and a TGF‐β‐resistant MME cell line (BG18.2). TGF‐β1 decreases newly synthesized cdc2 protein levels within 6 h after addition. Coincident with this decrease in newly synthesized cdc2 protein was a marked reduction in its ability to phosphorylate histone H1. This decrease in kinase activity is not due to a change in steady‐state levels of cdc2 protein, since mRNA and total protein levels of cdc2 are not reduced until 12 h after TGF‐β1 addition. This suggests that the kinase activity of cdc2 is dependent on newly synthesized cdc2 protien. Moreover, the protein synthesis of another cyclin‐dependent kinase, cdk2, is not effected by TGF‐β1 addition, but its kinase activity is substantially reduced. Thus, it appears that TGF‐β decreases the kinase activity of both cdc2 and cdk2 by distinct mechanisms.