Lymphocyte-enriched peripheral blood mononuclear leukocytes (PBL) were obtained concurrently from a group of gestating heifers at intervals throughout pregnancy and from a group of ovariectomized heifers of the same age. The cells were suspended in media containing either ovariectomized heifer sera or sera from heifers at the homologous stage of pregnancy, and incubated with various concentrations of Concanavalin A (Con A), Pokeweed Mitogen (PWM) or phytohemagglutinin (PHA). Their mitotic response to the lectins was then assessed by determining [3H]thymidine incorporation into DNA. The response of PBL from pregnant heifers gradually increased as pregnancy progressed when compared with the response of PBL from ovariectomized heifers. The increased response continued until parturition but was not detectable at two weeks postpartum. This pattern was demonstrated for all three mitogens, although the effect was more pronounced with Con A and PWM and PHA. Sera from mid-trimester pregnant animals suppressed the mitotic response of PBL from both pregnant and ovariectomized heifers. This suppressive effect was not detectable in sera from pregnant animals in early or late stages of pregnancy.