Mononuclear cell leukemia in aged Sprague-Dawley rats

A mononuclear cell leukemia in Sprague-Dawley rats is described in which liver, spleen, and lung involvement was a constant feature. The cell was 16 to 25 microns in diameter with round, oval or indented nucleus, and the cytoplasm contained bright red granules. This is the first report of such a leukemia in Sprague-Dawley rats and a comparison with mononuclear cell leukemia in other strains of laboratory rats is made.