PGE2treatment of mononuclear cells from patients with different types of neoplasias was unable to decrease either the number of plaque-forming cells or the expression of CD71 and CD25 in PWM-driven cultures. In contrast, in previous studies, PGE2inhibited these parameters in cultured mononuclear cells from normal volunteers. Surgical treatment of cancer patients did not modify the lymphocyte sensitivity to PGE2after 1 week, but at 2 and 6 months after therapeutical treatment, the inhibition values of the parameters studied were almost similar or very similar to those of normal lymphocytes. The reduction of PGE2sensitivity in cancer patients was related to the increase of PGE2levels and, probably, to a PGE2receptor saturation. A restoration of PGE2-induced inhibition some months after therapy could be due to the decrease in PGE2levels and to receptor unsaturation.