Indomethacin reversibly inhibits growth of normal (human fibroblast) and transformed (HTC) cells. Upon removal of indomethacin from HTC cells, there is a surge in DNA synthesis followed by an increase in mitotic index at 20 hours and cell number after 24 hours. A similar sequence of events was observed in fibroblast cultures. DNA cytofluorescent measurements and autoradiographic studies with labeled thymidine indicated that in the presence of indomethacin fibroblast cells were arrested in the G1 phase of the cycle and that once drug was removed all cells underwent mitosis. These data suggest that indomethacin arrests growth during the G1 phase of the cell cycle and upon removal the cells resume growth in synchrony.