We assessed the suppression of lymphocyte proliferation ovine endometrial cells recovered during each trimester (Days 45, 90, and 135) of pregnancy. We evaluated fractionated and unfractionated caruncular (C) and intercaruncular (IC) cells for suppression of cocultured peripheral blood lymphocytes (PBL) in phytohemagglutinin (PHA)-treated cultures. We also evaluated cells for the release of the suppressor factor into the culture medium and tested the factor for transforming growth factor-beta (TGF-beta) activity. Suppression of PHA-induced proliferation of PBL was evident for C and IC cells recovered from each day of pregnancy, and the activity was predominantly attributed to the population(s) of low-density (1.006-1.054 g/ml) cells. The activity was greater for unfractionated C than for IC cells on Day 45, whereas the pattern was reversed by Day 135 of pregnancy. For the C cells, the activity was greatest on Day 45 and least by Day 135. Although suppressor factor was released into the medium from cultured C and IC cells, its activity was not apparently mediated by TGF-beta. In conclusion, we observed a temporal pattern in suppressor activity for unfractionated endometrial cells during pregnancy. Suppression was predominately mediated by a population(s) of low-density cells. In addition, the cells released a soluble suppressor factor that seems to be unrelated to TGF-beta. The suppressor cells may provide immunological protection to the fetoplacental unit by suppressing specific lymphocyte responses directed toward conceptus tissues.