1 The reactivity of human peripheral blood mononuclear cells to phytohaemagglutinin ([3H]-thymidine incorporation) was enhanced in indomethacin- and eicosatetraynoic acid-treated cells compared with untreated cells, from normal volunteers. This suggests that endogenously synthesized prostaglandins have an inhibitory effect during cell preparation and/or culture. 2 Prostaglandin E2 inhibited [3H]-thymidine incorporation induced by suboptimal phytohaemagglutinin concentrations and had a more potent effect on indomethacin-treated cells than on untreated cells. 3 Prostaglandin I2 also exhibited an inhibitory effect and, under defined conditions, was more potent than prostaglandin E2 or than prostacyclin which had been allowed to decay at pH 7.4 and 37 degrees C. 4 These results indicate that, in attempting to define altered lymphocyte reactivity in disease states, the involvement of prostaglandins should be considered both during cell preparation and culture.