By the inoculation of the antigen prepared from the virus-infected animal organs into the hepatitis patients in Kagato district and the healthy men in other districts, a kind of allergic skin reaction was observed which were specific to a mild or subclinical infection of infectious hepatitis. In order to test the practical value of this skin reaction, the author performed many experiments and obtained the following results: 1) Of all the antigens prepared from various animal organs or tissues, the antigen from liver of the mouse inoculated with Ishihara strain showed the most effective result, and was used in the subsequent experiments. 2) In the forearm, 0.1 cc of the saline solution of antigen was inoculated intracutaneously. The test was tried on 375 hepatitis patients and 125 healthy men, and the frequency curve of the size of flush area was taken to estimate the standard size for the judgement of the result of skin reaction. That over 10 mm in diameter was determined as positive and that under 9.5 mm as negative. 3) The maximum of flush area was reached 20 hours after the inoculation of antigen and lasted till 28 to 32 hours, and it seemed reasonable to decide the result of reaction 24 hours after the inoculation. 4) As the disease became chronic, the specificity and appearance of reaction falled. 5) No noticeable side effect was produced by the inoculation of this antigen. 6) Judged from the frequency curve of the size of flush area, the specificity of the reaction seemed to be higher in the early stage of the disease than in the convalescent stage.