Transforming growth factor beta (TGFbeta) is essential for immune regulation and growth control. TGFbeta has previously been shown to inhibit cell growth as well as induce cell death in lymphocytes, although the mechanisms controlling these processes are not well understood. The mouse pre-B lymphoma cell line WEHI 231 was used to examine the mechanisms of TGFbeta-mediated growth inhibition and apoptosis. TGFbeta inhibits the growth of WEHI 231 cells in a dose-dependent manner; however, a decrease in viability is also observed, which is indicative of increased cell death. TGFbeta induces oligonucleosomal DNA ladder formation in a dose-dependent manner, indicating the mechanism of cell death is via apoptosis. The broad spectrum caspase inhibitor BD-fmk, but not tetrapeptide inhibitors of specific caspases, completely blocks TGFbeta-induced apoptosis while maintaining cellular viability. BD-fmk, however, has no effect on TGFbeta-induced growth inhibition. The ability of TGFbeta to down-regulate cyclin A-associated kinase activity, postulated to mediate TGFbeta growth inhibition, is not affected by the caspase inhibitor BD-fmk. These results suggest that caspases do not regulate cell cycle mediators of growth and provide the first demonstration that a pharmacological inhibitor of TGFbeta-induced apoptosis is capable of distinguishing growth inhibitory from apoptotic pathways.