Activin A, a member of TGF beta superfamily has various activities including induction of apoptosis in mammalian cells. However, it remains unknown how activin A induces cell death. To clarify this, we investigated the expression of BCL-2 and BAX, and the effect of BCL-2 overexpression on activin A-induced apoptosis in B cell hybridoma cell lines. The activin A-sensitive cell lines expressed BAX but not BCL-2 and that activin A did not increase BAX levels. Overexpression of human BCL-2 suppressed activin A-induced apoptosis in these cells. Thus, activin A has been shown to induce apoptosis by a BCL-2-inhibitable mechanism without activating BAX.