Summary: Typhus-yolk-sac vaccine-supernate (2–5) gave positive skin-reactions in high dilution on passively sensitized cutaneous sites prepared in normal test-subjects with sera from egg-sensitive individuals. The typhus-yolk-sac vaccine-sediment (2–5) also gave positive reactions, but of much lesser degree than the supernate (approximately 1/10th the reactivity). Whole typhus-yolk-sac vaccine (-7) gave reactions similar to those of typhus-yolk-sac vaccine-supernate (2–5). Comparative tests with the typhus-yolk-sac vaccine-supernate (2–5), normal yolk-sac, egg-white, and egg-yolk indicated that the vaccine supernate (2–5) contained at least 10 per cent of the reactivity of egg white, when compared on a protein-nitrogen basis. The normal yolk-sac reacted within the same range as egg-yolk and both were slightly less reactive than the vaccine-supernate (2–5). The highly purified rickettsial bodies diluted to 0.1 mg nitrogen/ml gave no skin,reactions in test-subjects sensitized with egg-sensitive sera. The reactions in such passively sensitized sites, showed that equine encephalomyelitic vaccine contained egg-yolk, yolk-sac as well as chick-embryo antigen, but very little egg-white antigen. Sensitivity was probably induced by injection of EE vaccine since the study of Nis serum showed that yolk-sac, egg-yolk, and probably chick-embryo were responsible antigens. There was no sensitivity to egg-white. Since EE vaccine, on first injection, may have induced sensitivity to egg-embryo antigens there may be potential danger of inducing sensitivity by the use of such immunizing agents. These results indicate that the injection of egg-sensitive individuals with typhus-vaccine similar to lot 2–5 might produce serious immediate systemic allergic reactions. The antibodies present in the serum of egg-sensitive individuals are not identical as shown by variations in neutralizing capacity of the different antigens. Qualitative differences in antibodies are present in the naturally sensitive man which are not apparent in artifical sensitization. Analogous differences have been previously noted, particularly in a study of ragweed- (23, 27) sensitive individuals.