Using a recombinant vaccinia virus expressing protooncogene Bcl-2, we demonstrate opposite effects of the expressed Bcl-2 in two cell lines: apoptosis induction in BSC-40 cells and apoptosis prevention in HeLa G cells. The apparent molecular weight of the expressed Bcl-2, its amounts and its effects on the mitochondrial membrane potential are comparable in both cell lines, suggesting that the consequences of Bcl-2 expression depend on the cellular environment. To further support these findings we demonstrate the pro-apoptotic effect of the expressed Bcl-2 in several other cell lines.