AbstractNovikoff rat hepatoma cells were propagated in suspension cultures containing 0.5 to 10 μC of3H‐methyl‐thymidine,3H‐5‐uridine,3H‐G‐adenosine or3H‐8‐adenine. The presence of the3H‐labeled precursors caused an inhibition of cell replication which was due to a delay or arrest of the cells in G2and M. The degree of inhibition was proportional to the amount of radioactivity incorporated into nucleic acids. Almost immediate and complete inhibition resulted from incubation with 10 μC3H‐thymidine/ml. The presence of 0.5 μC3H‐thymidine/ml caused a significant increase in the relative proportion of cells in G2+ M, even though the population doubling time of the culture