The issues of diagnosing the dog poisonings by isoniazid in the veterinary laboratories are considered in the article. The given type of research is of current interest because some years stray and decorative dogs are poisoned by the medicine for tuberculosis under the name of "Tubazid". Isoniazid (tubazid) is a medical product, antitubercular preparation, isonicotinic acid hydrazine derivative, intended for treatment of tuberculosis of all localization forms.