The relationship between the HEINZ granules and leucocytosis has been studied in dogs in which acetylphenylhydrazine has been administered. The HEINZ granules has been obtained injecting a well developed dog with high dose of acetylphenylhydrazine (70 mg per kilo body weight); the animal was killed by bleeding in the fifth day, the blood hemolised and the granules washed by centrifugalization with saline solution. The granules were then injected into small dogs, leucocytes counts determined each day. In one dog the leucocyte increased from 8.900 per cu.m to 20.000 per cu.m after 24 hours; in another dog from 16.000 to 35.800 per cu.m after 24 hours. The leucocytes number decreases in 3 to 5 days back to previous levels. It seems probable trat the substance which constitutes de HEINZ grande (denatured globin) is responsible for the leucocytes increases observed in th acetylphenylhydrazine intoxication.