We studied the effect of adriamycin on DNA synthesizing cells from the view point of cell cycle. DNA synthesizing cells were marked by bromodeoxyuridine, which is a known marker for such cells, before administration of adriamycin. Then, the level of this marker was sequentially measured by flow cytometry. At the low concentration (0.01 micrograms/ml), adriamycin caused delay of the shifting time to the S phase 16 hours after administration. The marked cells were accumulated at the G2M phase at a moderate concentration (0.1 micrograms/ml), and blocked at the S phase at the higher concentration (1.0 micrograms/ml).