Some Effects of Carcinogenic Agents on Mice Subject to Spontaneous Leukoses**

During the past 2 years we have been investigating the effect of painting with methylcholanthrene on the development of leukosis in dilute brown (strain Dba) mice (4, 5)The strain has been extensively studied by Little and Murray (3, 6, 7)" Although it is generally considered to have a high incidence of spontaneous mammary carcinoma, several sublines have been developed in which the tumor incidence varies. Line 212 has a relatively high incidence of spontaneous "lymphoblastoma" or lymphomatosis while in line i22 breast tumors predominate. Pulmonary tumors are rarely found. The exact incidence of spontaneous nonepithelial tumors is not known. They occur less frequently than in the C57 black mice. In the latter strain, investigated at the Roscoe B. Jackson Memorial Laboratory, i3.79 per cent of 877 mice developed nonepithelial tumors when 7oo to 75 o days old (3). Spontaneous lymphomatosis appears in dilute brown mice at an age of 65o to 8oo days at the Jackson Laboratory. It is found in the dilute brown stock at the New York State Institute for the Study of Malignant Disease in mice more than 5oo days old. The leukosis is characterized grossly by splenomegaly and enlargement of i or more groups of lymph nodes. In the spontaneous disease the mesenteric, retroperitoneal, and mediastinal nodes are involved more frequently than the cervical, axillary, or inguinal groups. The microscopic picture is that of leukemic infiltration of organs and tissues with cells resembling tymphobtasts. The original experiments were performed on a group of 5 o dilute brown mice obtained from the Jackson Laboratory. They represented a mixture of various