When human promyelocytic leukemia cell line HL-60 was treated with various differentiation-inducers, apoptosis always occurred after the full appearance of differentiation-related phenotypes. However, the two phenomena could be dissociated when HL-60 cells were treated with PDBu. When HL-60 cells were cultured with PDBu for more than 36 h, apoptosis was induced following differentiation. Apoptosis was not, however, observed when PDBu was removed within 24 h, even though induction of differentiation-related phenotypes, such as NBT-reducing ability and surface marker expression, was the same as that in the control. Northern blot analysis revealed that bcl-2 mRNA was rapidly down-regulated within 6 h of the treatment with PDBu. The amount of bcl-2 mRNA recovered to that of undifferentiated HL-60 cells when PDBu was washed out within 24 h. In contrast, the recovery of bcl-2 was incomplete when the cells were treated with PDBu for more than 36 h, suggesting that bcl-2 is also a critical regulator of the cell fate during myeloid differentiation. This hypothesis was confirmed by experiments using antisense oligonucleotides, i.e., blocking the recovery of bcl-2 mRNA by antisense oligonucleotides could result in the induction of apoptosis in HL-60 cells from which PDBu was removed within 24 h. Moreover, overexpression of BCL-2 in HL-60 cells could block apoptosis during differentiation without any significant effect on differentiation itself. These results strongly suggest that apoptosis is not a simple consequence of differentiation-induction, and that apoptosis and differentiation are regulated independently in myeloid cells.