A guinea pig skin assay was developed as a measure of ileal loop fluid-inducing factor in the rabbit and possibly of the causative principle of Bacillus cereus food poisoning. A green or bloody necrotic reaction was produced in the guinea pig skin by injection of culture filtrates from 21 of 24 B. cereus strains tested, and by no other Bacillus species tested except for the closely related B. thuringiensis. The skin factor was synthesized and excreted by logarithmically growing cells, inactivated by heating at 56 C for 5 min, precipitable by ammonium sulfate, and was nondialyzable. The skin activity was not related to the lecithinolytic or hemolytic activities of B. cereus. Production of the skin factor depended on the medium in which the culture was growing. The factor was an antigenically active substance. There are indications that at least two antigenically distinct species of skin factors exist.