1. Six adult swine were given phenylhydrazine hydrochloride orally and intravenously. Hematologic observations, which included periodic bone marrow studies were made before, during, and after the administration of the drug.
2. Five swine responded to the drug in the usual manner with progressive anemia, reticulocytosis, and erythrocytic hyperplasia of the bone marrow. Three animals died between the thirty-ninth and fiftieth day of the experiment after receiving a total dose of 0.30 to 0.35 Gm. per kilogram of body weight. Two swine survived and recovered after receiving a similar dose.
3. One animal died on the fourteenth day of the experiment and exhibited a course which closely resembled that of benzol poisoning. Rapid and progressive granulocytopenia, anemia, and extreme universal hypoplasia of the bone marrow were observed.
4. A direct correlation between the mean corpuscular volume of the red cell and the per cent reticulocytes was found within the limits of 0 to 80 per cent reticulocytosis. With each increment of 10 per cent in the reticulocytes the mean corpuscular volume increased approximately 6.8 cubic microns.