Effect of glucuronic acid on the toxicity and antigenity of diphtheria toxin in vitro was examined and its reaction mechanism was considered. Toxicity of this toxin weakened or disappeared easily on treatment with glucuronic acid but not its antigenity. Consequently, this change is not accompanied by marked degeneration of the protein and may be termed as the formation of a toxoid. The weakening or loss of toxicity by glucuronic acid treatment is not recovered by dialysis to remove excess glucuronic acid so that the change cannot be considered as the formation of a mere mixture or a salt, the product being a comparatively stable substance. Since the toxicity and antigenity of diphtheria toxin are lost by its treatment with 2, 4-dinitrofluorobenzene, free amino acid is thought to take part in the toxicity and antigenity of this toxin. It was found by treatment with dinitrofluorobenzene to form dinitrophenylamino acid that ε-position in lysine had a free amino group. Therefore, weakening and disappearance of toxicity of this toxin by treatment with glucuronic acid is thought to be the result of reaction between glucuronic acid and free amino group in the toxin protein molecule, i.e. formation of N-glucuronide. Toxin free of toxicity by treatment with glucuronic acid was found to contain a definite quantity of glucuronic acid which could not be removed by salting out or dialysis.