Immune reactivity in lymhoid organs of rats during the last week of syngeneic (Lewis: L x L) or allogeneic (females Lewis x males Wistar: L x W) pregnancy was compared with that found in non-pregnant animals. The thymus weight was slightly reduced and the response of thymic lymphocytes to phytohaemagglutinin and concanavalin A was slightly, but not significantly, elevated in pregnant rats. By contrast, the response of lymphoid cells from spleen and mesenteric lymph nodes to the mitogens was reduced in rats during advanced pregnancy. The immune response of lymphocytes from pregnant L x W rats to allogenic (Brown-Norway: BN) or semiallogeneic (W) irradiated cells was tested by the mixed lymphocyte culture (MLC) assay. The MLC response of para-aortic lymph nodes towards the unrelated BN cells was elevated over that of non-pregnant rats on day 15 of pregnancy. No significant enhancement was observed at the same time in the MLC response of mesenteric lymph nodes. On day 20 of pregnancy, a reduced MLC response towards BN cells was found in the mesenteric and para-aortic lymph nodes. On day 15 of pregnancy, the MLC response of mesenteric and, more markedly, that of para-aortic lymph nodes to paternal (W) cells was enhanced, as compared to that of non-pregnant rats. On day 20 of pregnancy, the response of mesenteric lymph nodes was suppressed, while the response of para-aortic lymph nodes was similar to that found in non-pregnant rats. Since the latter lymph nodes are the most directly exposed to antigenic stimulation from the uterus, it seems unlikely that the suppression in response to T-cell mitogens observed in the spleen and mesenteric lymph nodes during pregnancy accounts for the survival of the foetus. It seems more plausible that local factors in the vicinity of the uterus protect the foetus from being rejected.