Studies of the effect of pregnancy on the capacity of mice to resist Listeria monocytogenes and Toxoplasma gondii infection revealed significantly diminished resistance of pregnant mice to infection by both agents as measured by mortality. The development of immunity to listeria was assessed by studying the kinetics of listeria growth in the livers and spleens of virgin and pregnant mice infected intravenously with a sublethal dose of listeria. In both virgin and pregnant mice there was a rise in the number of listeria colony-forming units per organ during the first 3 days after infection. Thereafter, there was a decline in colony-forming units in these organs in virgin mice but a persistence of listeria in spleens and livers of pregnant mice. Paradoxically, during the first 3 days after infection, listeria counts in spleens of virgin mice were significantly higher than those in pregnant mice. Nonspecific resistance to listeria conferred by chronic infection with T. gondii was significantly diminished in pregnant mice when measured by mortality and quantitative cultures of listeria in livers and spleens. These studies demonstrate a remarkably decreased resistance of pregnant mice to two intracellular organisms and a diminished capacity of pregnant mice to develop immunity to listeria. This decrease in resistance may play an important role in congenital transmission of these organisms.