It was found that pregnant mice exhibit increased susceptibility to lethal action of Listeria monocytogenes, although there was no correlation between increase of susceptibility of animals to lethal effect of bacteria and weakening of elimination of microorganisms from the spleen. Increased susceptibility of animals to listeriosis was accompanied by weakening of some parameters of cellular immunity. In pregnant mice of strains C57Bl/6, naturally resistant to infections, inhibition of II-1 by macrophages was found which at the same time exhibited depression of chemotaxis. Weakening of these macrophage functions did not influence the development of delayed hypersensitivity to Listeria antigen (LA) in C57Bl/6 mice--the reaction which was suppressed in pregnant mice of A/J strain which is characterized by natural susceptibility to listeriosis. Inhibition of development of delayed hypersensitivity to LA in A/J pregnant mice was caused by defective function of dendritic cells and T lymphocytes. Dendritic cells of A/J pregnant mice exhibited limited ability of presentation of LA to immune T lymphocytes isolated from animals infected with L. monocytogenes or L. innocula. On the other hand, immune T cells of A/J mice cultured with LA exhibited inhibition of proliferation and production of MIF.