Increasing evidence suggests that gamma interferon (IFN-gamma) is the major or sole factor in human lymphokines which activates blood monocyte-derived macrophages (M phi) to inhibit or kill Toxoplasma gondii and certain other intracellular pathogens. In the current studies, we found that IFN-gamma effectively activated tissue M phi from adults (peritoneal M phi) and from newborns (placental M phi) as well as blood-derived M phi from adults and from newborns to kill or to inhibit the replication of T. gondii. Results with purified and recombinant IFN-gamma and with adult and newborn M phi were similar. IFN-gamma-treated M phi were equally or more active against T. gondii than were freshly isolated monocytes and M phi. Recombinant IFN-alpha A and IFN-beta were less effective than IFN-gamma. IFN-gamma also inhibited survival and replication of T. gondii in WISH cells more effectively than did IFN-alpha and IFN-beta. These findings are consistent with an important role for IFN-gamma in the control of Toxoplasma infection and indicate that the anti-Toxoplasma activity of resting and IFN-gamma-activated adult and neonatal M phi is similar. The increased susceptibility of neonates to T. gondii is not due to a defect in M phi effector function.