Biological properties of endotoxins prepared from three strains of Vibrio parahaemolyticus were compared with reference to commercially prepared Salmonella typhi endotoxin. Endotoxin assays performed in rabbits included dermal Shwartzman reactivity, pyrogenicity, heat stability, and ability to induce tolerance as well as cross-tolerance.Mice were used for endotoxin LD50 determinations. Results showed V. parahaemolyticus endotoxins were similar to that of S.typhi strain 0901.Induction of tolerance to V. parahaemolyticus strain 11590 endotoxin resulted in complete cross-tolerance to S. typhi endotoxin, and vice versa. Partial cross-tolerance to S. typhi endotoxin was demonstrated with rabbits rendered tolerant to endotoxin from V. parahaemolyticus strains Sak-3 and FC 1011. Absorption spectra, nitrogen, phosphorus and carbohydrate analyses revealed additional similarities between endotoxins from V. parahaemolyticus and endotoxin from a member of the Enterobacteriaceae.Known in Japan for two decades as a causative agent of large outbreaks of seafoodassociated poisoning, Vibrio parahaemolyticus has recently been found associated with similar outbreaks in the U.S.A. [1,4,7,8,14,26,40,42,43], and other countries [2,3]. Nevertheless, the mode of pathogenesis of V. parahaemolyticus is poorly understood. Strains isolated from clinical specimens and displaying hemolysis on selected media (Kanagawa-phenomenon positive) are considered to be enteropathogenic [30,39,49], whereas nonhemolytic (Kanagawa-phenomenon negative) seafood or seawater isolates are generally considered non-enteropathogenic [19,37,38]. The problem is further complicated by the frequent isolation of several different serotypes of V. parahaemolyticus during an outbreak of food poisoning [7,8].Biologically active components (hemolysins) from selected strains of V.parahaemolyticus have been isolated which do not, however, account for the spectrum of gastroenteritis symptoms observed in clinical cases [12,17,19,25,28,29,31,36,50,51]. A single case of endotoxin shock and leg gangrene due to infections by V. parahaemolyticus following an attack of gastroenteritis has been demonstrated [36].Endotoxins from V.parahaemolyticus have not previously been studied, although some chemical analyses ofO antigens [44,45] and K antigens have been reported [32,33]. The purpose of this research was to study the biological properties of V.parahaemolyticus endotoxins and to determine if they are similar to endotoxins classically prepared from selected members of the Enterobacteriaceae, such as Salmonella typhi.