The heat-labile toxin (HTL) was purified from sonic extracts of Bordetella bronchiseptica and Bordetella pertussis cells by a series of hydrophobic interactions, density gradient centrifugation, gel filtration, isoelectric precipitation, and isoelectric focusing. A 114-fold purification was regularly obtained with a yield of 31%. A dose of 0.75 ng was dermonecrotizing in guinea pigs. HLT is a simple protein (pI 6.9) with a molecular weight by gel filtration of 102,000 which consists of two polypeptides of 30,000 and 24,000 molecular weight. Amino acid analysis showed 15 common amino acids and the absence of methionine. The dermonecrotizing activity was inactivated at 56 degrees C, and at above pH 10 or below pH 5. Effects of various ions, detergents, enzymes and other chemical agents on HLT were determined. When instilled on the surgically exposed peripheral blood vessels of guinea pigs or suckling mice, HLT induced vasoconstriction within 15 min resulting in the decrease of blood flow, followed by manifestations of ischemia, diapedesis and petechial hemorrhage during the following 5 h. HLT activity on arterioles was unaffected by adrenergic or cholinergic blockades. Biochemically, HLT significantly inhibited in vitro the activity of Na+ - K+ ATPase prepared from rat kidney. A possible mechanism by which HLT induces dermohemorrhagic necrosis and splenic atrophy, is discussed.