The classical enterotoxins are known primarily for their ability to cause emesis and diarrhoea in cases of staphylococcal food poisoning but they also exhibit other biological activities. The seven antigenic types of toxin have molecular weights in the range 25 000-35 000. All types induce emesis in man and monkey and are of comparable potency. The enterotoxins seem to induce emesis by stimulating neural receptors in the intestine rather than acting on the medulla directly. The mechanism whereby diarrhoea is produced is unclear. Another product of Staphylococcus aureus which meets the more recent definition of an enterotoxin is the delta toxin. This toxin is an amphipathic peptide having an Mr of 2977 and possessing the ability to interact with a variety of hydrophobic substances. It is cytotoxic, can increase vascular permeability in guinea-pig skin, and can increase cellular cyclic AMP levels in guinea-pig ileum. In the ileum delta toxin also inhibits water absorption, apparently by increasing the bidirectional movement of Na+ and Cl- across the mucosa. This response does not appear to be mediated by cyclic AMP since the changes in ion fluxes precede the increases in cellular cyclic AMP levels. In high doses delta toxin also elicits a positive response in the neonatal mouse after intragastric inoculation.