In the course of the studies on acquired sensitivity of rabbits to different organisms, previously reported (1), it had been observed that normal animals showed definite inflammatory responses to intradermal injections of bacterial suspensions of comparatively low concentration. The organisms employed were laboratory strains of B. typhosus, B. paratyphosus A, B. coli, Streptococcus hemolyticus and Staphylococcus albus that had been heated for one hour at 60°C. before use. If, for example, a skin sensitivity test, consisting of intradermal injections of 0.1 cc. quantities of suspensions containing varying numbers of organisms per cubic centimeter, was administered to a normal rabbit, inflammatory responses might have appeared in the course of twenty-four hours in the areas wherein the suspensions containing 1,000,000,000, 100,000,000 or perhaps 10,000,000 organisms per cubic centimeter, had been injected. These inflammatory responses were at first believed to be the result of endotoxins contained in the suspensions.