A purified tetanus toxin containing 0.023 X 10-6 mg. N2 per mouse M.L.D., a better purification than any other reported in the literature, has been prepared. A potent strain of Clostridium tetani is grown in a medium containing excess iron, and the toxin produced is separated by suitable precipitations. The yield of the purifying procedures is approx. 50% of the original amt. of toxin, the loss occurring apparently because of incomplete elution rather than destruction of the toxin. There is no evidence that the limit of purification has been achieved, and although all data so far obtained are compatible with a protein toxin, the possibility remains that the toxic substance is a non-protein compound.