A short‐term in vitro method was employed to study the Mitomycin‐C sensitivity of normal mouse bone marrow CFU without triggering the G0‐phase cells into the proliferative cycle. Comparison was made of the toxicities of the drug against cells in different phases of the cell cycle including G0. Mitomycin‐c killed CFU both in and out of the S‐phase. No significant difference could be found between its toxicities against normal and proliferating CFU; along the exponential part of the survival curve 1·6 μg/ml concentration of the drug reduced survival to 10%. Although in the normal bone marrow only a few CFU are in the S‐phase and are killed by the agent, presence of the sensitive G0 cells produce a significant amount of non‐S‐phase mortality. Among the proliferating CFU population the non‐S‐phase lethality is less due to the absence of G0 cells. About 75% of the S‐phase cells are killed after incubation with 1 μg/ml drug; outside the S‐phase, the lethality is about 40–50%. The studies indicate that the G0 cells which are situated near the G1‐S boundary are almost as sensitive to the drug as other non‐S‐phase cells like G1 cells. The clinical significance of the findings is discussed.