Two basic glycoproteins (UTM-P) with molecular weights of 57,000 and 59,000 were purified from ovine uterine milk collected on Days 125 and 130 of pregnancy. The UTM-P were evaluated for immunosuppressive activity in phytohemagglutinin (PHA)-treated, mixed lymphocyte (MLC) and resting lymphocyte (RLC) cultures. For PHA and RLC cultures, UTM-P (2.5 to 800 micrograms UTM-P/ml) were added to 1 X 10(6) lymphocytes and 0.8 micrograms of PHA (for PHA cultures only), while for the MLC, UTM-P (50 to 1600 micrograms UTM-P/ml) were added to 5 X 10(5) lymphocytes combined from each of two ewes. Following [3H] thymidine addition, cells were later harvested for determination of thymidine incorporation. Lymphocyte blastogenesis was suppressed by UTM-P in PHA (R2 = 0.32 to 0.92, P less than 0.01 to 0.001), MLC (R2 = 0.8, P less than 0.001) and RLC (R2 = 0.65, P les than 0.01) experiments. To determine reversibility, PHA-treated lymphocytes were incubated with UTM-P for 6, 12 or 24 h, then washed to remove surface UTM-P. Incubation was continued in the presence of PHA as with other experiments. Exposure of lymphocytes to UTM-P for 6 or 12 h did not result in suppression of blastogenesis, whereas exposure for 24 h was sufficient for suppression (P less than 0.01). In an additional experiment, UTM-P were added to PHA-treated cultures at 0, 6, 12 or 24 h. Suppression (P less than 0.01) of blastogenesis was observed for each time period. Immunosuppressive activity was not mediated by overall cytotoxicity and was not affected by routine handling and storage of UTM-P. Data from these experiments provide one explanation for tolerance of the conceptus allograft during defined stages of ovine pregnancy.