I. In previous reports (1), a toxin producing strain of staphylococcus has been described. This strain, “Ha,” produces in 10 day broth cultures a filterable substance that kills rabbits in twenty-four hours or less. The m.l.d. is about 0.1 cc. per kilogram of body weight. Whole cultures kill in a similar fashion. Burnet (2) has studied the lethal properties of similar strains and in particular, one strain, “Wood.” Filtrates of broth cultures of this strain kill rabbits in about five minutes after the intravenous injection of small amounts. A sublethal amount of filtrate, injected intracutaneously, produces an antiserum that neutralizes some, if not all, of the toxic properties of the filtrate. It is the belief of Burnet (2) that intracutaneous immunization produces the most potent antiserum. Further to confirm or disprove the findings of Burnet and to determine the precipitating and protective properties of filtrate Ha antiserums, the following studies were undertaken.