A variety of cyclooxygenase (FCO) and 5-lipoxygenase (5-LPO) inhibitors were tested for their ability to modulate murine thymocyte proliferation induced by IL-1 and suboptimal levels of the mitogen phytohemagglutinin (PHA). The contribution of drug toxicity to inhibition of 3H-thymidine incorporation was estimated by measuring MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) formazan production in the thymocyte cultures at the end of the assay. Cyclosporin A and dexamethasone, two positive control compounds, potently inhibited thymocyte proliferation at extremely low concentrations (0.01 and 0.001 micrograms/ml respectively), although activity roughly paralleled toxicity. In contrast, 5-LPO inhibitors (AA-861, BW-755c, and ETYA), but not selective FCO inhibitors (ibuprofen and indomethacin), suppressed lymphoproliferation at nontoxic concentrations, suggesting that products of the 5-LPO pathway may mediate the thymocyte proliferative response induced by IL-1/PHA. Attempts to counteract the suppressive activity of 5-LPO inhibitors by addition of leukotriene (LT) B4, LTC4, LTE4, 5-HETE, and 15-HETE were unsuccessful.