AbstractA phagocytic cell of the thymic reticulum in culture (P‐TR‐C) has recently been isolated whose characteristics closely resemble those of the thymic interdigitating cell despite the modifications induced by culture conditions.In the present report it is demonstrated that this cell population is able to produce both interleukin 1 (IL1) and prostaglandin E2(PGE2). IL1 has no direct mitogenic effect on thymocytes but is comitogenic with concanavalin A. PGE2has no direct effect on the secretion of IL1 by P‐TR‐C, but has an effect antagonistic to IL1, inhibiting thymocyte proliferation. P‐TR‐C are thus able to modulate thymocyte proliferat