Endotoxins have been reported to occur inAspergillus fumigatus andA. flavus, but the data regarding them are incomplete. Optimal cultural conditions and methods of extraction are described here for the production of the toxins of both of these fungi. They are obtained from the well-washed mycelia; only minute amounts could be demonstrated in the concentrated culture fluids. Both are powerful nephrotoxins; a characteristic necrosis of the kidney cortex is evident grossly in mice, and histological study shows no difference between the effects on the kidney of the two toxins. However, the fumigatus extracts have strong hemolytic properties, especially for sheep erythrocytes, and marked dermonecrotic activity in rabbit skin. No hemolytic action has been demonstrated for the flavus extracts, which possess a powerful hemagglutinin for rabbit erythrocytes and only mild dermonecrotic properties. Immune sera have been prepared in rabbits for both toxins, and in each case these prevent all the observed effects of the toxin. Precipitin titers are closely correlated with neutralizing titers. Chemical differences are suggested by the different procedures applicable to concentration and purification of the toxins. The toxin fromA. fumigatus is precipitated by acetone and can be concentrated and purified to a considerable degree by acetone precipitation and dialysis. The flavus toxin has not been precipitated by any of the solvents tried but can be freed to some extent of inert substances by acid treatment and saturation with ammonium sulfate.