Sulphadimidine, dapsone, and pyrimethamine have been tested alone and in various combinations for their therapeutic effect against toxoplasma infection in mice. In the treatment of active infection, sulphadimidine by itself was effective, but relapses were common. Pyrimethamine gave complete cures and prevented the carrier state when used in doses near to the toxic level. Dapsone alone was not as good as either of the other two drugs tested. The best combination was found to be sulphadimidine and pyrimethamine, which were synergic. In doses well below the toxic level, this combination not only controlled the acute infection but also prevented relapses and the development of the carrier state. Dapsone and pyrimethamine were also synergic, but were not as effective as the previous combination. No synergy was found between dapsone and sulphadimidine. The mechanism of relapse and the development of the carrier state and the modes of action of the drugs alone and in combination are discussed.