Summary An antigen prepared from adult worms of Schistosoma japonicum by Melcher's method was injected intradermally into 392 primary school children and 401 junior high school students in an endemic area where the S. japonicum infection rates, determined by repeated fecal examination by the MIFC technique, were 15% for boys and 7% for girls. The skin reaction was observed at 15 minutes after injection and the area of wheal, as well as its diameter, and the zone of erythema were measured. For this mixed population of infected and noninfected individuals, the frequency distribution curve of the size of the skin reaction showed a bimodal character regardless of reading methods. The first peak is attributable to the physiologic reactions (negative reaction), whereas the second peak is due to specific reactions (positive reaction) to the antigen in individuals with present or past infections. Therefore, the boundary between positive and negative reactions in this study was chosen at the site of the trough of the bimodal distribution curve which was 0.7 cm2 for the area of wheal, 9 mm for the wheal diameter, and 20 mm for the erythema diameter. Analysis of the skin reactions to a series of twofold dilutions of antigen indicates that the skin reacts to a specific antigen according to the all-or-none law, so that a threshold of minimal concentration of antigen for a positive skin reaction can be determined for each individual. The threshold of skin sensitivity for individuals passing eggs of S. japonicum is mostly lower (smaller protein nitrogen content of antigen in the solution) than it is in individuals who are not passing eggs. According to this mode of reaction, the exact amount of injected antigen is considered to be not so critical.