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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.

From t'he Departments of Anatomy, Radiology, and Phgsiology,S ~'~t i r c r s i t y of MinnesotaThe percutaneous administration of either 9,10dimethyl-l,2-benzanthracene or methylcholanthrene was reported by Engelbreth-Holm and Lefbvrel and Engelbreth-Holm* to accelerate the development of mammary cancer in female mice of strain Dba. Morton and Mider3v4 had previously treated similar mice in like manner, but had not described this phenomenon. The latter authors stated specifically3 that male and female animals were segregated, whereas Engellbreth-Holm made no definite statement in this regard. This possible difference in technic suggested to the present authors that breeding might condition the response to carcinogens insofar as development of mammary cancer was concerned. Another explanation for the discrepancy in the above findings could be th: use of different sub-lines of this strain of mice in the two investigations. I t was therefore decided to test the effect of breeding on the carcinogenic induction of mammary 'cancer within a single sub-line of strain Dba.O I t was first demonstrated that 23 out of 26 untreated breeding females of the sub-line