Small amounts of PGE inhibit mitogen-induced [3H]thymidine incorporation in human peripheral lymphocytes. The 50% inhibitory concentration is approximately 10(-7) M, and this is reduced to approximately 10(-8) M when endogenous PGE production is blocked. PGE inhibits PHA- and Con A-stimulated cultures much better than PWM cultures, suggesting a differential effect of PGE on T-cell vs. B-cell function. In vitro blockade of PG synthesis results in approximately 50% increase in [3H]thymidine incorporation in PHA cultures. PGE is produced endogenously in PHA cultures by glass adherent suppressor cells.