The changes in immune system of pregnant murine and postnatal development of offspring's immune system dams after single administration of concanavalin A (a T-cell mitogen) were investigated. The administration of concanavalin A on the 7th day of pregnancy led to accelerated involution of thymus and sharply increased proliferation of splenocytes in murine dams and did not change the pregnancy duration and outcome. The stimulation of maternal immune system in early pregnancy with Concanavalin A prior to formation of fetal immune system resulted in impaired development of the offspring's spleen and thymus, that can have a long-term consequences.