Toxoplasma gondii, an obligate intracellular parasite, is readily grown in nearly all cultured cells. The host-parasite relationship in these cultures can often be explored by using mutant host cells or mutant parasites. Host cells incapable of incorporating uracil or hypoxanthine, which were excellent precursors for T. gondii, allowed the demonstration that the host cell had no access to the purine or pyrimidine nucleotide pools of the parasite. Conversely, a T. gondii mutant that was defective in the principal pyrimidine salvage pathway allowed the demonstration that the parasite had no access to pyrimidine nucleotide or deoxynucleotide pools of the host cell. One metabolite that must pass from the host cytoplasm to T. gondii is a purine. An absolute defect in purine biosynthesis by the parasite was disclosed by growing T. gondii in a mutant host cell that was, itself, incapable of purine synthesis. T. gondii grew normally at 40 degrees C in a mutant host cell that was incapable of protein synthesis at that temperature. Thus, the parasite did not depend on concomitant protein synthesis in the host cell. An antigenic mutant of T. gondii was isolated with the aid of parasiticidal monoclonal antibody. This mutant lacked a major parasite surface protein, of relative molecular mass (Mr) 22 000. The antibody used to select this mutant immunoprecipitated a protein of this Mr from the wild-type parasite.