To each of the rabbits immunized with egg-white albumin or bovine serum the egg-white albumin and bovine serum are injected, and guinea pigs are passively sensitized by injecting the blood drawn periodically from each group of these rabbits. By observing the appearance of skin anaphylaxis in these guinea pigs the author obtained the following results. 1. The titer of antibody in circulating blood rapidly decreases immediately after the injection of antigen, but gradually recovering thereafter, it returns to the pre-injection level in one week. 2. The residual antigen in circulating blood increases rapidly after the injection of antigen, but it then decreases abruptly one day later, disappearing completely one week afterwards. 3. The desensitization of Arthus phenomenon is completely established within five minutes after the intravenous injection of antigen. 4. It seems that there is generally a parallel relationship between the intensity of Arthus phenomenon and the antibody titer of blood. 5. The residual antigen in the blood seems to inhibit the appearance of Arthus phenomenon.