In this study, we examined the effects of ultraviolet-B (UV-B) irradiation on a variety of cultured cells including human (HL-60, U937) and mouse (MI, WEHI3B/D) leukemia cells, and mouse macrophages (P388-D1, J744). UV-B irradiation for 15 min variably induced apoptosis in all of the cell lines examined, except for J744. Apoptosis was apparently prevented by the treatment of a panel of anti-oxygen reagents. WEHI3B/D, M1 and HL-60 induced cell differentiation showed delayed induction of apoptosis. Protein kinase C and bcl-2 protein expression did not change during the process of apoptosis. These results indicate that (1) UV-B irradiation induces apoptosis of leukemia cells and macrophages via direct and/or indirect DNA damage, and (2) cell differentiation results in less frequent apoptosis.