As observed previously with hepatoma cell cultures, nonsteroidal anti-inflammatory drugs inhibited proliferation of human fibroblast cells in culture. The order of potency of these drugs in inhibiting growth was similar to that reported for their anti-inflammatory activity. The drugs did not alter cell viability, and their action was reversible. After a short delay (18 hr), exponential growth resumed once the drug was removed from the culture. These kinetics suggest the possibility that the cells had been arrested in the G1 phase of growth. No evidence was found that the anti-inflammatory drugs acted through inhibition of prostaglandin synthesis. Exogenous prostaglandins did not reverse the inhibition, and prostaglandin synthesis could not be detected in the cultures. BIBLIOGRAPHIC REFERENCES: Hial, V., Horakova, Z., Shaff, R.E. and Beaven, M.A.: Alteration of tumor growth by aspirin and indomethacin: Studies with two transplantable tumors in mouse. Eur. J. Pharmac. 37: 367-376, 1976.