The effect of interferon on the exponential growth phase of BALB/c 3T3 cells was studied. Although interferon reduced the growth rate and the proportion of cells in both the M and S phases (mitotic index and labelling index), there were no appreciable differences in the duration of these phases between control and interferon-treated cells. Moreover, the shape of the first peak of the fraction of labelled mitoses (FLM) curve was not altered by treatment with interferon, which indicates that the duration of the S and G2 phases was not affected. However, the height of the second peak of the curve in interferon-treated cells was extremely reduced as compared to control culture. These results are compatible with the idea that the suppressive effect of interferon is exerted mainly in the G1 phase (A-state) of the cell cycle of BALB/c 3T3 cells.