The author made experiments on the professional allergic dermatitis with benzoyl chloride. The results were as follows. (1) The serological chemical properties of benzoylated proteins were recognized in the rabbit serum by applying repeated an soy-bean oil solution of benzoyl chloride (10%) on the rabbit skin. Positive effects to induce hypersensitiveness of the skin with benzoyl chloride in the same rabbit were obtained. (2) The rabbit received intravenous injections of a solution containing 0.2% of benzoylated human sera were tested 3 weeks after the 7th injection. It was obserbed that the benzoylated human sera antirabbit serum had a chemical specificity of benzoyl chloride which reacted with benzoylated proteins. (3) Positive effects of hypersensitiveness on the skin with benzoyl chloride were obtained to the same rabbit sensitized by intravenous injections of benzoylated human sera. (4) Hypersensitiveness of the skin was confirmed on the guinea-pigs sensitized actively by intracutaneous injections with benzoyl chloride.