Type A toxin used in these experiments was produced by growing CI. botulinum in tubes of beef heart medium for ten days under a vaseline seal. The supernatant fluid from these cultures was centrifuged at high speed for one hour which removed most of the organisms. The rabbit was given an injection with 0.1 gm. of heparin in 5 cc. of sterile (0.85*^) salt solution and placed under ether anesthesia. The small intestine was exposed and placed in warm gauze packs which were kept moist with warm salt (0.859^) solution. A suitable loop of small intestine was selected and the vein coming from it was cannulated, but the artery was left intact. The loop was then carefully ligated and the toxin injected in the end nearest the stomach. The point of injection was touched with tincture of ferric chloride and this portion of the loop was ligated. The loop was carefully manipulated so as to distribute the toxin throughout its length. The animal was kept alive by frequent blood transfusions, using heparinized blood from normal rabbits. Samples of blood were collected at five minute intervals and injected into mice in 0.5 and 0.1 cc. amounts. At frequent intervals, also, mice were given injections with 0.5 cc. of blood and 0.25 cc. of the homologous antitoxin.