The manifestations of toxoplasmosis depend to a great extent on the number and virulence of the organisms reaching the tissues, the reaction of these tissues to the organisms, and the ability of the cells of the affected host to overcome the proliferating organisms. Despite a considerable amount of work on the virulence of various strains of Toxoplasma and their interactions with antibody and the host cells of various species1-6much needs to be done before we can completely comprehend the pathogenesis of congenital and acquired toxoplasmic infections. There has been a good deal of interest in the past few years concerning the morphology of Toxoplasma, its methods of propagation, and the reasons for variation in virulence of strains. A large number of puzzling observations have been made with regard to variation in virulence; for example, Lainson4recovered low-virulence strains from rabbits, which barely produced infections in mice on