Soluble toxin of H. lineatum injected i.v. induced a severe cardiorespiratory shock syndrome in calves. Intradermal injection of the toxin caused well-defined, edematous wheals comparable with those produced by histamine. Treatment of calves with famphur (organophosphate insecticide; poured on the skin of the calves 24 h before injection of toxin) did not increase the severity of the effects of the toxin. Treatment of calves with phenylbutazone (20 mg/kg) 20 min before injection of toxin protected the calves against the systemic shock syndrome and the local inflammatory reaction. The neurotoxic mechanism of the organosphosphate insecticide is independent of the action of the Hypoderma toxin per se. Phenylbutazone can control the effects of the Hypoderma toxin.