A systematic histologic and cytologic study of Rauscher disease was undertaken. After inoculation with Rauscher virus, 184 BALB/c and 93 C57BL/6 mice were randomly killed at uniform intervals. Additional lots of inoculated mice were subjected to repeated hematologic examinations until death. Also, 32 Wistar rats were inoculated and observed until death. The disease in BALB/c mice began with a proliferation of young cells of the erythroid series and of primitive reticulum cells in the spleen and liver. In a second phase, a leukemia of myeloblastic type, beginning in the marrow, was observed in most of the animals surviving the first phase. The initial leukemic lesions before generalization were revealed only by systematic cytologic studies and differential counts of the marrow. No lymphatic leukemia was observed and the thymus was generally normal, except for its occasional, secondary invasion by myeloblastic cells. In C57BL/6 mice, splenic and hepatic erythroblastosis was absent, but there was regularly a myeloblastic leukemia of medullary origin (occasionally chloroleukemia). Only in Wistar CF rats was a lymphatic leukemia of thymic origin observed. The first phase of Rauscher disease in BALB/c mice is identical with the first phase of Friend disease. Rauscher disease and Friend disease seem to make up a particular group among the murine leukemias.