The effects of N4-behenoyl-l-~-~-arabinofuranosylcytosine (BH-AC) on the cell cycle of murine leukemic cells (L 1210 cells) were compared with those of 1-8-D-arabinofuranosylcytosine (ara-C), known to be effective for acute leukemia. In a cytokinetic study, a combination of Feulgen microcytofluorometry and tritiated thymidine autoradiography (3H-TdR ARG) was used to measure DNA content and to determine DNA synthesis simultaneously in a single cell. Administration of 200 mg/kg of BH-AC significantly prolonged the survival time of mice bearing L 1210. In addition, the cells in the G , +S1 phases increased with time, accounting for 94.4 per cent of all cells measured at 48 h after the administration, compared to 73-7 per cent before administration. On the other hand, following the administration of 86mg/kg of ara-C (equivalent to 200 mg/kg of BH-AC), the percentage of cells in the S, + S , phases increased maximally to 75.9 per cent at 18 h. Cytokinetic studies further showed that BH-AC administration blocks DNA synthesis of the cells in the S-phase for a longer period than does ara-C. These results suggest that the prolonged inhibition by BH-AC on DNA synthesis allows cells to accumulate in the S-phase to a greater degree.