Observations were made on viable lymphocytes in 22 specimens of blood from 20 patients to test whether leukemic lymphocytes have distinctive characteristics. Special slide-chambers were used in this study. The features suggestive of leukemic lymphocytes were: large chromatin masses, prominent nucleoli, abnormal size of cells, flagella-like structures, reduced motility, and an increase in the sensitivity to inactivated guinea pig sera. An index summarizing these abnormal features selected correctly 9 blood specimens from 7 patients with chronic lymphocytic or lymphosarcoma-cell leukemia. Distinctive abnormalities of the living lymphocytes were not found in the blood of 3 patients who were diagnosed lymphosarcoma and had normal lymphocyte counts and some lymphosarcoma-cell lymphocytes in stained blood films. Blood lymphocytes from 8 patients with infectious mononucleosis, diabetes, hemochromatosis, and other diseases were correctly identified as non-leukemic. The findings obtained in this study indicate that viable leukemic lymphocytes are characterized by abnormal morphologic features, reduced motility, and increased sensitivity to heat-inactivated guinea pig sera.