Previous data have shown that lymphocytes from pregnant women, but not from non-pregnant individuals, displayed progesterone receptors. These receptors are inducible in normal human lymphocytes in vitro by mitogenic or allogeneic stimuli. The present study was designed to test the role of in vivo allogeneic stimulation in inducing progesterone receptors in lymphocytes from transplanted and transfused patients. Receptors were detected by immunohistology using a progesterone receptor-specific MoAb and avidin-biotin system. Peripheral blood lymphocytes from 56 healthy pregnant women, 8 liver-transplanted patients and 15 transfused patients contained significantly more receptor-positive cells (P less than 0.001) than those of non-pregnant individuals. In transplanted and transfused patients no correlation was found between the percentage of positive lymphocytes and age, sex or transplant survival. Our results show that in these three groups the percentage of receptor-bearing lymphocytes was higher than in normal subjects.