Summary The higher the Lf-units of diphtheria-toxin or anatoxin, the smaller the Kf-value, as has been asserted by many observers. If diphtheria toxin or anatoxin is freed from accompanying substances by dialysis or purification by the calcium-phosphate method, its Kf-value becomes smaller. The Kf-value of diphtheria-toxin or anatoxin largely depends on its pH. Irrespective of the kind of culture media in which toxin was produced, it was found that (a) the Kf of the purified and its original toxin is least in the untreated solution and increases with the amount of acid or alkali added. (b) When purified toxin was mixed with buffer solutions of different pH, the Kf was found to be the smallest in the mixture with the buffer of pH 6. (c) Both original and purified anatoxin had the smallest Kf-value in the most acid range within which flocculation did occur. Anatoxin prepared from semi-synthetic broth toxin the Kf of which was found ∞, showed flocculation when it was dialysed, acidified or purified.