Mice infected with live Listeria monocytogenes intravenously from 1 week to 3 months following splenectomy exhibit greatly enhanced antibacterial resistance to this micro-organism as compared to normal or sham-splenectomized mice. They survive a dose of Listeria 100 times higher than is the LD50 of this parasite for normal mice. Initially, the same number of viable micro-organisms lodge in the livers of splenectomized and normal hosts. However, within 24 h after infection, the number of viable Listeria which can be recovered from the livers of splenectomized animals is significantly reduced in comparison with control mice. This effect of splenectomy is transient and gradually disappears spontaneously within 3 months following splenectomy. Enhancement of anti-listerial resistance in splenectomized mice can be abrogated by the transfer of normal spleen cells. The presence of a normal splenic cell population that controls macrophage activation is postulated.