An isoniazid-resistant strain of avian mycobacteria was shown to have an enzyme which decomposed isoniazid to hydrazine and isonicotinic acid. This enzyme was extracted to cell-free extract and enzymologic studies were carried out. It was concluded that this bacterial enzyme is quite different from liver transhydrazidase and is probably an enzyme of the hydrolytic type, group-specific for acid hydrazides. It has been tentatively named "hydrazidase.".