The in vitro and in vivo effects of indomethacin on blastogenesis of rat splenic lymphocytes in response to phytohemagglutinin (PHA) were investigated. Direct addition of indomethacin to lymphocyte cultures was found to have both a stimulatory effect at low concentrations (less than or equal to 0.003 microM) and an inhibitory effect at higher concentrations (greater than or equal to 0.01 microM). The in vitro stimulation was significant only at submaximal and supramaximal concentrations of PHA, whereas the inhibitory response was observed with a wide range of mitogen concentrations. When indomethacin was administered to animals twice daily for 3 days, similar dose-dependent stimulatory and inhibitory effects were observed. Again the stimulatory effects were associated with lower doses (0.1 microgram/kg) and were found to be significant only with submaximal PHA concentrations. The inhibition with higher doses of indomethacin (IC50 = 0.20 mg/kg) was accompanied by a dose-dependent decrease in the maximal response and an increase in the EC50 to PHA in indomethacin-treated animals. These inhibitory effects of indomethacin administration on lymphocyte proliferation were found to occur at doses which closely approximate those required for the anti-inflammatory effects of the drug.