In vitro effects of the Bordetella HLT on the isolated perfused lung and some other tissue preparations from guinea pigs were examined. When HLT (30 to 300MNDs/ml) was administered, an increase of the perfusion pressure was induced in the perfused lungs, indicating vasoconstriction. When 100 or 300MNDs/ml of HLT was given, the pressure increase appeared after a lag period of 3.5 to 4min, reached a maximum within 8 to 13min, and then slowly decreased by 60 to 80% 25min after exposure. In calcium-free medium, the pressure increase dur to HLT did not occur, but these HLT-treated lungs manifested an increase without any lag period immediately after the calcium-free medium was replaced by normal medium containing calcium. No difference in the response of the perfused lungs to histamine was observed before and after exposure to HLT. The arterial strip did not respond to HLT, but after predigestion with a collagenase and elastase solution the contractive response to 100MNDs/ml of HLT appeared with a lag period of 1min. HLT had no effect on the pharmacological responses of the isolated atria, deferent canal or intestinal preparations, or on the ciliary movement of cultured tracheal rings.