Peripheral blood and splenic lymphocytes from interstrain (L X BN) or intrastrain (L X L) primigravida rats were equivalent to those from virgin L females in their in vitro DNA synthetic responses to paternal strain cells (BN), to unrelated allogeneic cells (ACI), and to the mitogen phytohemagglutinin (PHA). Heat-inactivated serum from pregnant L rats, when compared to serum from virgin or postpartum L rats, regularly suppressed the in vitro response of L lymphocytes to paternal and allogeneic cells. The response of L cells to PHA was not suppressed. The degree of inhibition was related to the final concentration of pregnant serum in culture, concentrations above 2% producing more than 80% inhibition of the mixed lymphocyte reaction. The inhibiting sera were not cytotoxic by a sensitive 51Cr release assay. Histoincompatibility between mother and fetus is not required for production of this inhibitory effect, since it is consistently present in intrastrain (L X L) pregnant rats. Among interstrain pregnant rats, the degree of inhibition is influenced by, but not specific for paternal-strain alloantigens. Thus, the proliferative function of thymus-derived (T) lymphocytes from pregnant rats is intrinsically normal, but the response to allogeneic cells can be altered by factors present in pregnant serum.