Apoptosis was induced by treatment of HL-60 cells with C2-ceramide. Apoptotic damage of DNA was detected according to the sub-Gl peak on a flow cytometer, according to the typical morphology and according to the DNA fragmentation "ladder" after gel electrophoresis. It was shown that the apoptotic cleavage followed after G1 blockade of the cell cycle. A high correlation coefficient (rs=0.957) was found between the percentages of G1 blocked cells and apoptotic cells. This high correlation together with the appearance of the sub-Gl peak suggests that the G1 blocked HL-60 cells were subject to apoptotic death. It is deduced that the mechanisms leading to G1 blockade of the cell cycle and activation of apoptosis in HL-60 cells are interconnected.