Experimental animals infected with avirulent strains of toxoplasma usually harbor cysts of this organism in their brains. The infection may be transferred to other animals by the injection of brain cysts but this technique possesses the disadvantage of difficulty in quantitating the inoculum. Trophozoites of avirulent strains of toxoplasma can be maintained easily in almost any type of tissue culture. Thus an ideal, constantly available supply of countable numbers of parasites may be used for experimental purposes. Using such methods, infections were established in mice and guinea pigs with avirulent toxoplasma grown in homologous tissue culture. Following infection both species of host rapidly developed immunity to a highly lethal challenge with a virulent strain of toxoplasma. Animals that survive the acute stages of infection with Toxoplasma gondii probably remain infected for life. A high degree of immunity develops in chronically infected hosts that is manifested by specific resistance to lethal challenge doses of virulent strains of toxoplasma. Specific immunity to toxoplasma has been demonstrated in a number of species of laboratory animals including rabbits (Laveran and Marullaz, 1913; Huldt, 1966), hamsters (Frenkel, 1967), guinea pigs (Cutchins and Warren, 1956), and mice (Weinman, 1943; Jacobs, 1953; Frenkel, 1956). A broad spectrum of parasite strain virulence and varying levels of host susceptibility in different laboratory animals are encountered in experimental toxoplasma infections (Jacobs, 1953; Frenkel, 1953). Thus several methods of obtaining chronically infected animals are available. In certain hosts such as the guinea pig (Huldt, 1963; Foster and McCulloch, 1968) low doses of a virulent toxoplasma strain allowed a majority of the animals to survive. Virulent strains of toxoplasma may even be used to establish chronic infections in such susceptible hosts as the mouse Received for publication 30 July 1970. * This investigation was supported by Grant No. 5-R22-AI08608-01 from the United States Japan Cooperative Medical Science Program administered by the NIAID of the NIH, Department of Health, Education, and Welfare. t Present address: Palo Alto Medical Research Foundation, 860 Bryant St., Palo Alto, California 94301. and hamster if drug treatment is employed. La ge doses of sulfonamides administered early in the course of infection allow animals to survive the acute phase of the original toxoplasma infection and subsequently resist a highly lethal ch llenge (Weinman and Berne, 1944; Frenkel, 1956). Relatively avirulent strains of toxoplasma can also be used to establish chronic infection and subsequent immunity. Following i oculation of infected brain tissue, which contains toxoplasma cysts, a prolonged chronic infection is established. However, the latter method contains at least two disadvantages: (1) quantitation of cysts and the number of p rasites they contain is difficult, and (2) repeated passage of such brain cysts tends to increase the virulence of the strain (Jacobs and Melton, 1954). Virulent and avirulent strains of toxoplasma have been maintained for prolonged periods in tissue culture without undergoing "autosterilization" (Schuhova, 1960; Bickford and Burnstein, 1966). In this report an avirulent strain of toxoplasma was maintained in and harvested from guinea pig kidney cell and mouse L-cell tissue cultures. Trophozoites obtained from these cultures were used to determine the usefulness of the tissue culture method in immunizing mice and guinea pigs with known numbers of an avirulent strain of the organism. Immunization of these animals with avirulent toxoplasma trophozoites cultured in vitro conferred an early onset of resistance to lethal toxoplasma challenge.