When Toxoplasma gondii is grown in the peritoneal cavity of white Swiss Webster mice, an exudate forms that is rich both in parasites and white cells. After the removal of all of the cells from the exudate, the resulting fluid produces lethal convulsions when injected into the tail veins of mice. Our studies have demonstrated that these lethal effects can be positively correlated with the viscosity and surface-tension characteristics of this fluid. Our studies also suggest that the observed lethal effects of supernatant fluids from Toxoplasma-induced mouse peritoneal exudates may be attributed to protein-mucopolysaccharide complexes in the exudate, rather than to a specific toxin elaborated by the Toxoplasma organisms.