mcl-1 was identified as an "early-induction" gene that increases in expression during the differentiation of ML-1 human myeloblastic leukemia cells. The mcl-1 gene product proved to be a member of the bcl-2 gene family and, like bcl-2, to have the capacity to promote cell viability. The pattern of expression of mcl-1 has now been characterized, the aim being to determine whether increased expression is consistently associated with differentiation-induction and whether expression is also associated with other changes in proliferative state or cell viability. Expression of the mcl-1 mRNA was found to increase rapidly in ML-1 cells exposed to inducers of monocyte/macrophage differentiation (phorbol esters or lymphocyte conditioned medium), but not cells exposed to an inducer of granulocyte differentiation (retinoic acid). Expression also increased rapidly in response to certain cytotoxic agents (colchicine and vinblastine), but did not increase during serum stimulation or growth-arrest in reduced serum. Increased expression of mcl-1 occurred during the initiation of cell differentiation or death and was not inhibited by cycloheximide, in agreement with the designation of mcl-1 as an early-induction gene. Increased transcription contributed to the increase in expression, and turnover of the mcl-1 mRNA was rapid. These findings suggest that mcl-1 may serve as a modulator of cell viability that can undergo rapid upregulation as well as downregulation, with upregulation harbingering the initiation of cell differentiation or death.