Human macrophages derived from in vitro culture of peripheral blood monocytes were studied under a variety of conditions to determine their microbicidal capacity for the obligate intracellular protozoan, Toxoplasma gondii. The effect of macrophages on intracellular Toxoplasma was evaluated morphologically by light and phase microscopy and by autoradiography. When macrophages from dye test (DT)-negative or DT-positive individuals were infected with Toxoplasma in the presence of normal human serum, the organisms were able to multiply intracellularly with resultant destruction of the monolayer. Once organisms were intracellular, the presence of antibody-containing serum in the medium did not alter this inability of the macrophages to kill Toxoplasma. However, when Toxoplasma were incubated in the presence of heat-inactivated DT-positive serum just before infection of the monolayers, the intracellular organisms were inhibited or killed by normal macrophages. Attempts were made to activate macrophages in vitro to kill Toxoplasma. Macrophages incubated in the presence of sensitized lymphocytes and Streptokinase-Streptodornase (SK-SD) or Toxoplasma lysate antigen (TLA) were found to kill Toxoplasma when compared to macrophages incubated in the presence of lymphocytes from DT-negative individuals and TLA or lymphocytes alone. Thus, in vitro induction of resistance (both specifically and nonspecifically) in human macrophages was accomplished by culturing these cells in the presence of specifically sensitized lymphocytes and antigen. These results suggest that, as in the mouse model, activated human macrophages have the ability to inhibit or kill intracellular Toxoplasma and that these cells may be important as effector cells in cell-mediated immunity (CMI) to toxoplasmosis in man.