THE NEED for a simple and specific technique for demonstrating and assaying staphylococcal enterotoxin has been recognized for a long time. Research efforts ranging from biological tests with small and relatively cheap animals (1, 2) to extensive chemical and serologic studies of enterotoxin (3) have not attained this important objective. The many contradictory reports found in the literature (4) on staphylococcal enterotoxin are evidence of the inadequacy of the crude, difficult, and impractical tests that are available for its detection. Evidence incriminating suspected foods in outbreaks of staphylococcal food poisoning is largely circumstantial and is limited to the use of epidemiological findings and the demonstration of the presence in the suspected food of appreciable numbers of enterotoxin-producing staphylococci. The very ubiquity of the staphylococcus and, conversely, the possibility of the presence of the heatresistant enterotoxin in foods which no longer contain viable staphylococci, detract considerably from the value of such procedures. Furthermore, the demonstration of enterotoxigenicity of the isolated staphylococcus involves considerable effort. The isolated organism must be cultured on special media in order to produce the enterotoxin, and the presence of the latter is determined by the feeding of monkeys or the parenteral introduction of the culture filtrates into monkeys or cats. When available, human volunteers may be fed the suspected food or the culture filtrate. Monkeys, cats, and humans vary considerably in their susceptibility to the enterotoxin and may acquire an increased tolerance to it. Prior to parenteral administration of the culture filtrate it is necessary to remove or neutralize the alpha or beta hemolysins which may be present. These toxins are lethal and may in themselves elicit the emetic reaction characteristic of the enterotoxin. The monkey-feeding test, although specific, is impractical because of its low sensitivity, the marked variation in susceptibility of the animals, and obvious problems of their cost, availability, handling, and naintenance. Parenteral administration of the enterotoxin to cats is complicated by the activity of the alpha and beta hemolysins, by rapid production of increased tolerance to the enterotoxin, and by a considerable variation in susceptibility of test animals. This method is, however, more sensitive, cheaper, and more convenient than the monkey-feeding test. In the studies presented here, 3 to 10 ml. of culture filtrate was injected intravenously into unanesthetized adult cats, as described by Hammon (5). Prior to injection, alpha and beta hemolysins were removed by boiling, neutralization with antiserum containing antibodies for alpha and beta hemolysins, or by digestion with pancreatin (unpublished data). The cats were used once only. Key strains of staphylococci, selected for the production of enterotoxin for immunization purposes, were checked for enterotoxigenicity by feeding culture filtrates to monkeys. The studies described here were designed to Dr. Casman is a bacteriologist in the Division of Microbiology, Food and Drug Administration.