The author studied chemo-immunological specificity of iodated protein and phenylureido protein as well as sensibilization of iodine and phenylisocyanate and obtained the following results. 1) Iodated egg-albumin antibody reacts with iodated human serum albumin antigen and this reaction is inhibited by diiodotyrosine. Phenylureido egg albumin antibody reacts with phenylureido human serum albumin antigen and this reaction is inhibited by phenylureidolysine. 2) According to quantitative preicipitin reaction, in the inhibitory reaction of the monovalent antigen (semihapten) mentioned above, the quantity of precipitin nitrogen decreases in proportion to the increase in mono-valent antigen. 3) Iodated egg-albumin antibody reacts with iodated human-serum albumin antigen by the agar method of Oudin, and phenylureido egg-albumin antibody reacts with phenylureido human-serum albumin antigen. The number of the immunological reaction ring due to chemoimmunological specificity proves to be single. 4) The guinea pigs sensitized by iodated human-serum albumin demonstrate anaphylaxis by intravenous injection of iodated egg-albumin, and guinea pigs sensitized by phenylureido egg-albumin develop Arthus reaction by intracutaneous injection of phenylureido human-serum albumin. 5) When guinea pigs are sensitized subcutaneously with phenylisocyanate, they develop Arthus reaction by phenylisocyanate. Therefore, this is a substance that possesses sensibilization. On the other hand, in the case where Arthus reaction is made to take place by phenylureido human-serum albumin in the guinea pig previously sensitized by phenylureido egg-albumin, when the mixture of phenylureido human-serum albumin and phenylureido-lysine is injected, the reaction is weakened. Namely, there can be observed an inhibitory reaction by monovalent antigen. 6) From these results, it is assumed that allergic skin inflammation and anaphylaxis in the animals sensitized by iodine and phenylisocyanate is due to the antigen-antibody reaction of the determinant groups as diiodotyrosine and phenylureido-lysine respectively.