Female BALB/c mice were tested during the first week of pregnancy for their lymphocyte-mediated cytotoxic response to paternal alloantigens. Spleen or uterine regional lymph node cells were not spontaneously cytotoxic against concanavalin A-activated paternal target lymphocytes. Female mice immunized i.p. with paternal H-2-matched or third-party allogeneic cells on the fifth day and tested on the 12th day of pregnancy demonstrated total suppression of cell-mediated cytotoxicity to paternal alloantigens and partial suppression to third-party alloantigens. A generalized non-specific immunosuppression to alloantigens seems to be associated with pregnancy, which may indicate that soluble factors were involved in mediating the suppressive effect. Cocultures of spleen cells from virgin mice and the whole population of spleen or regional lymph node cells from allogeneic pregnant female mice demonstrated specifically suppressed responses to alloantigens. Similar cocultures with Thy 1.2- and Lyt 2.2-depleted populations restored the cytotoxicity levels of activated spleen cells. We conclude that antigen-specific Lyt 2+ T cells were activated during pregnancy to regulate the female T-cell response to paternal alloantigens.