Prostaglandins (PGs) have an important physiological role in the modulation of various cell immune functions. The main sources of PGs during immune responses are monocyte cells. We report here the ability of non-stimulated macrophages to synthesize prostanoids and show that peritoneal mouse macrophages synthesize PGE 2 , PGF 2a and thromboxane B 2 , spleen macrophages produce PGE 2 and PGF 2a , and in a fresh medium this synthesis reaches a constant basal level in a few hours. We studied the kinetics of Con A-induced proliferation of murine splenocytes under the influence of a wide range of PGE 2 concentrations (10 14 -10 7 M). The suppressive effect of PGE 2 decreased when its concentration was lowered and disappeared at 10~9 M PGE 2 (this concentration corresponded to the basal level of non-stimulated macrophage synthesis of PGE 2 ). Further lowering of the concentration became essential for the proliferation process once again, and at picomolar concentrations PGE 2 caused a suppressive effect comparable with that for 10 8 M PGE 2 . We also found that PGE 2 significantly inhibited cell proliferation when it was added 1 h before the addition of mitogen, as compared with simultaneous mitogen addition. The effect was obtained for both low (

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