Characteristics of protective mechanisms during pregnancy were investigated using neonatally thymectomized (NTx) and/or pregnant mice infected with sublethal doses of Listeria monocytogenes, of which the explosive growth at an early phase of 2 or 3 days after infection is prevented by non-immune macrophages, and complete elimination at a late phase from 4 to 10 days after infection is attributed to the augmented functions of macrophages in co-operation with lymphokine-producing sensitized T lymphocytes. Although in virgin control mice there was a gradual decline of bacteria from the day after infection, viable bacteria in pregnant mice showed an increase in number until Day 3. In such pregnant mice, carbon clearance was suppressed. Thus, the enhanced bacterial growth in pregnant mice within 3 days may be attributable to the suppressed functions of non-immune macrophages. Complete elimination of Listeria from Day 4 was observed in pregnant sham-operated mice as well as in non-pregnant and pregnant NTx mice. Twenty-four hour reaction of delayed-type in normal mice induced by sheep red blood cells (SRBC) in incomplete Freund's adjuvant (IFA) was not affected by pregnancy, while 48 hr reaction in mice immunized with SRBC in complete Freund's adjuvant (CFA) was suppressed by pregnancy. We have reported previously that macrophage migration inhibitory factor (MIF) was produced in the latter but not in the former, and that the tuberculin type of delayed hypersensitivity accompanied by MIF production scarcely participated in acquired resistance to Listeria. Effective elimination of Listeria in pregnant and/or NTx mice at a late phase may be attributable to the activity of cellular immunity comparable to 24 hr reaction. These results suggest that T cells showing a low degree of thymus dependency in the ontogenic development may be the major component required for acquired protective immunity against Listeria and may account for the protection in pregnant mice.