Immune reactivity of primiparous pregnant C57Bl/6J mice was investigated using in vitro assays of mitogen reactivity. The response to the T cell mitogens phytohemagglutinin (PHA) and concanavalin A of cells from the paraaortic (PA) lymph nodes, which drain the uterus, was decreased in pregnant animals. Reactivity to lipopolysaccharide, a B cell mitogen, was normal. The decreased PHA response was seen with PA cells from mice bearing syngeneic or allogeneic (to DBA/2J) fetuses. It was not due to a change in sensitivity to PHA dose or to active suppression (as demonstrated by mixing experiments). Phytohemagglutinin reactivity of cells from inguinal nodes of pregnant mice showed a more variable depression of response in comparison to that seen with cells from the draining PA nodes. The response of axillary and brachial node cells was similar to virgin values. Statistical analysis revealed no differences in the average number of PA lymphocytes or fetuses per mouse between mice bearing syngeneic or allogeneic fetuses. This parallels the similarities found between syngeneic and allogeneic matings in in vitro functional assays. This study demonstrates that pregnant mice (syngeneic or allogeneic) show only a decrease in T proliferative capacity localized to the area of the uterus, while such responses in the rest of the body are left essentially intact.