Summary and Conclusions: Certain strains of staphylococci produce a filtrable substance lethal for rabbits that is not the result of bacterial autolysis or the decomposition of the protein constituents of the media. The lethal factor is produced in cultures independently of the production of hemolysins. Less of hemolytic activity is associated with an increase in toxicity. The hemolysin is unstable while the lethal factor maintains its toxicity for periods as long as one year without refrigeration. A discussion of the association of source of strains, lethal factor production and types of lesions produced by staphylococci is included. The nature of the toxin and its similarity to tuberculin is also discussed. It is suggested that the toxin-producing properties of unknown strains of staphylococci can be foretold by the degree of initial acidity and end alkalinity attained in broth cultures.