Incubation of RF/UN murine myeloid leukemia cells for 60 minutes with 0.3 microgram/ml cytosine arabinoside (ara C) inhibited the incorporation of 3H-TdR in 8 of 10 experiments by 17-90%. Incubation with 3 microgram/ml ara C inhibited DNA synthesis in all experiments by 36-99%. This inhibition of DNA synthesis was highly reversible with the effects produced by the high drug concentration being less reversible than the inhibition produced by 0.3 microgram/ml drug. The same exposure to ara C had little or no killing effect on the colony-forming cells (CFUC). Incubation with ara C and 3H-TdR did not reduce the 3H-TdR suicide index (S.I.). Incubation with ara C for 24 h produced a less reversible inhibition of DNA synthesis and resulted in the death of 60% of the clonogenic cells.