Hairs of the Thaumetopoea pityocampa caterpillar (Lepidoptera) cause cutaneous reactions in men and animals. A soluble substance extracted from the hairs has been shown to cause a reaction in guinea-pig skin, probably caused by mediators released by mast cells. A direct effect of this substance on mast cells has been shown. Degranulation of mast cells was found to be dose-dependent. Heating of this substance greatly reduced its effects. Proteins fractions are currently being extracted and will be submitted to the same tests.