Summary: Methods are given for the accurate and reproducible assay of the biological properties of tetanal toxin and tetanal toxoid. The solubilities of toxin and toxoid, and the optimal points for their precipitation in methanol-water mixtures under controlled conditions of pH, ionic strength and temperature have been determined. Tetanal toxin is quantitatively precipitated at a pH 5.1 ± 0.1 in 40 per cent methanol at ionic strength of 0.09 and at a temperature of −5 C. MLD and Lf concentrations of over 300 times greater than the original crude toxin on a nitrogen basis and of over 5000 times on a volume basis have been obtained. A purified toxin prepared from a special toxin (Mueller) contained 16,000,000 MLD and 2400 Lf units per mg of nitrogen, and further purification of this material has resulted in a crystalline product containing 75,000,000 MLD per mg of nitrogen. Tetanal toxoid is quantitatively precipitated at a pH between 4.8 and 4.9 in 40 per cent methanol at ionic strength 0.09 and at a temperature of −5 C. Lf concentrations of 400 times on a nitrogen basis over the crude toxoid have been obtained. On a volume basis concentrations of over 5,000-fold over the original crude toxoid are possible. The purified toxoid is relatively free of sensitizing material as indicated by anaphylactic studies. I am indebted to Miss Ruth G. Wittler and Mr. Donald B. Grossberg for their help in carrying out assay and analytical procedures. Wyeth Incorporated generously supported this work.