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We had earlier shown that human foetal epithelial cells (WISH), growth-inhibited by interferon γ (IFNγ), were reversibly detained at a point prior to DNA synthesis. In the present study, we determined the window of action of IFNγ in the G1 phase duration and the exact point of detention of WISH cells in cell cycle progression with respect to the known points of detention by the inhibitors of DNA replication initiation (aphidicolin and carbonyl diphosphonate) and of activation of replication protein A (6-dimethylaminopurine), of which RPA activation being the earlier event compared to DNA replication initiation in cell cycle progression. WISH cells, which were released from IFNγ-induced arrest, permeabilised and exposed independently to these inhibitors show that IFNγ detains WISH cells prior to initiation of DNA synthesis. Further, exposure of IFNα-synchronized (at G0/G1) or mimosine-synchronized (at G1/S) WISH cells to IFNγ, which was added at different time points post-release from the synchronizing agent, showed that the cells were promptly responsive to the growth inhibitory action of IFNγ only during the first 11 h in G1 phase. Taken together, these results suggest that IFNγ inhibits growth of WISH cells by detaining them at a point prior to initiation of DNA synthesis and that the IFN acts within the first 11 h in G1 phase of the cell cycle.