In this study we present evidence for reactivity of pregnancy lymphocytes, but not nonpregnancy lymphocytes, with the progesterone receptor-specific monoclonal antibody mPRI. Using an avidin-biotin peroxidase detection system, we found a nuclear staining in 14.6 +/- 3.7% (mean +/- SEM, N = 27) of pregnancy lymphocytes, while only 0.47 +/- 0.33% (mean +/- SEM, N = 15) of nonpregnancy lymphocytes reacted with the antibody. To characterize the receptor-bearing subset, CD8+ and CD4+ cells were depleted by complement-dependent lysis. Depletion of CD8+ cells was accompanied by 62 +/- 18% loss of progesterone receptor-bearing cells, while depletion of CD4+ cells resulted in a twofold increase in the number of positively staining lymphocytes. In nonpregnancy lymphocytes a 3-day PHA treatment, as well as allogeneic stimulation, resulted in a significant increase in the number of receptor-containing cells. These results suggest that pregnancy, but not nonpregnancy, lymphocytes contain progesterone binding structures, and that these are inducible by mitogenic or alloantigenic stimuli.