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A toxin associated with Toxoplasma gondii infection was obtained from the trophozoites and culture medium used to propagate the parasite in cell cultures. The toxin, named Toxofactor (TF), administered parenterally or nonparenterally in adult mice, produces transient symptoms of lethargy, ruffled fur, and body weight loss. Organ changes which accompanied the outward symptoms included hepatosplenomegaly and involuted thymus. TF activity was detected in extracts of the blood, peritoneal fluid, liver, and spleen of infected mice. Severe damage to embryonal and fetal development was induced when TF was administered during pregnancy. Resorption, abortion, and congenital abnormalities were produced, dependent upon the stage of development at the time of exposure. Adult mice which had reacted to and recovered from an initial intraperitoneal injection to TF were protected against a secondary challenge from TF. Fetal development was also protected from damage when TF was used to challenge adults previously exposed to TF. Mouse and rabbit anti-TF sera neutralized TF activity in the adult. In no instance did control mice show any deleterious effect when exposed to soluble cell lysate from the uninfected cell line (BHK-21) used to propagate the organism plus the used medium from these same uninfected cells. TF activity was not attributed to bacterial, myocoplasmal, or viral contamination. TF toxic activity is labile to elevated temperature and high or low pH, which also destroy its protective properties. TF activity was sensitive to trypsin and was obtained in the elution fraction (alpha-methyl-D-mannoside) from affinity chromatography (concanavalin A-Sepharose 4B). Ultrafiltration indicated the molecular weight to be between 50,000 and 100,000. TF, apparently a glycoprotein, was quantitated for activity by a weight loss assay. A unit of activity was defined as the minimum quantity of TF (highest dilution) which produced at least a 10% average body weight loss in adult Nya:NYLAR female mice between days 7 and 12 post-intraperitoneal injection.