Ramon1 proposed the use of tetanus toxoid (anatoxin) for immunization against tetanus. He prepared the toxoid by adding from 3 to 4 cc. of formaldehyde to a liter of tetanus toxin, and then placing the mixture in an incubator at 37 C. for four weeks. In this manner he detoxified toxin that would kill a 300 Gm. guinea-pig in a dose of 0.0001 cc, so that from 5 to 10 cc. injected subcutaneously into a normal guinea-pig would have no effect. When the tetanus toxoid was injected subcutaneously into guineapigs?three doses of 1 cc. each were given at intervals of two weeks? the animals showed a high degree of protection against the injection of multiple fatal doses of the unaltered tetanus toxin. Guinea-pigs could withstand as many as from 10,000 to 15,000 M.L.D. of tetanus toxin one month after they had been immunized by injections of three doses of 1 cc. of toxoid.