1. The minimum amount of diphtheria toxin which killed normal mice of from 24 to 30 gm. in weight upon intravenous injection, was found to be between 75 and 100 times the M.F.D. for the guinea pig. When given intraperitoneally, the fatal dose for mice was as high as 200 M.F.D.
2. There was no significant difference in the lethal action of diphtheria toxin for normal mice and mice in which an elimination of the reticulo-endothelial system had been attempted by means of blocking injections of India ink, or splenectomy, or a combination of both operations.
3. Attempts to infect normal mice and mice treated as described with large doses of a highly virulent diphtheria strain were unsuccessful with both groups of animals.