A variant of a standard strain of Cl. tetani has been obtained which is capable of producing an unusually high titer of toxin in the presence of high levels of iron on a medium composed mainly of hog-stomach autolysate and beef-heart infusion. This toxin can readily be converted to toxoid with formalin, and the toxoid is antigenic for mice roughly in proportion to its test-dose. Somewhat less potent toxoid prepared on an almost identical medium has for some time been used for human immunization without producing untoward reactions (4). One may therefore assume that there should be no drawback in the use of the more potent material beyond that theoretically inherent in the use of enzymic digests generally. The high level of antigenicity should render such toxoid admirably suited to use in conjunction with gas-gangrene or diphtheric toxoid, since it could be diluted out ten times or more and still remain equivalent to commercial toxoids now being distributed. Both the toxin here described, and the toxoid prepared from it flocculate sharply and rapidly with tetanal antitoxin, the former at approximately the neutral point. The reaction is entirely comparable with that of potent diphtheric toxin. The toxin should consequently be of considerable value in making possible the rough appraisal of antitoxic horse-sera during the course of immunization. The ultimate objective of this work is to produce an equally potent toxin on a chemically defined medium. This may well prove to be a difficult matter and will certainly not be quickly accomplished. In the meantime it is hoped that the possibilities of this remarkably powerful toxin will give the work some immediate practical value. The strain of Cl. tetani is, of course, available to any laboratory.