Although PGE1, in pharmacologic quantities, possesses significant in vivo immune-modulating activities, its effects on different lymphoid populations have not been defined. In the present study, lymphocyte numbers, turnover rates, and functional activities were assessed in Swiss-Webster mice receiving PGE1, 200 microgram twice daily, for up to 14 days. The most pronounced cellular alterations were observed in the thymus. After 2 wk, the number of theta-positive cells decreased by 81%. Total splenic cellularity was concomitantly reduced by 28%. This was primarily due to a loss of B lymphocytes; the number of these cells decreased by 40%. Bone marrow lymphocytes were also decreased by PGE1 administration. By contrast, peripheral blood lymphocytes and splenic T cells were not significantly altered. Similar changes in lymphocyte numbers were observed in adrenalectomized mice, indicating that depletion was not due to a corticosteroid effect. In normal mice, PGE1 reduced the in vivo incorporation of 3HTdr in both the thymus and the spleen by approximately 50%; this finding, in conjunction with histologic and cytologic observations, suggests a decrease in intrinsic rates of lymphopoiesis. Residual splenocytes were not impaired in their in vitro responses to three polyclonal mitogens: PHA, Con-A, and endotoxin. Studies in cortisol-treated mice showed that PGE1 caused a decrease in both steroid-sensitive and steroid-resistant thymocytes and splenocytes.