The effects of interferon-α (IFN α) treatment on following heat-induced apoptosis were evaluated in human myeloid leukemia cell line, U937. Incubation of U937 with 1000 U/ml of IFN α promoted heat-induced apoptosis in a dose- and time-dependent manner. Another human leukemia cell line, HL60, was also sensitized to heat-induced apoptosis by pretreatment with IFNα. To investigate the mechanism of sensitization, effects of IFNα on the expression of HSP70 and BCL-2, two major anti-apoptotic proteins, were evaluated. Both protains were upregulated by heat treatment, but irrespective of the presence or absence of IFNα. These results suggests that IFNα promotes heat-induced apoptosis in human myeloid leukemia cells through the mechanism whereby HSP70 or BCL-2 is not involved.