Two patients with typical chronic lymphocytic leukemia received a continuous infusion of 3H-thymidine for 7 days. The production rate of lymphocytes was low in the lymph nodes, the marrow, and the blood. The fraction of newly formed lymphocytes appearing in the blood per day was only about 0.5%. The absolute production rate of blood lymphocytes however was very high, 690 cells/cu mm blood/day in one patient and 725 cells/cu mm/day in the other. The daily accumulation of newly produced lymphocytes in the circulating blood was estimated to be 3.0 x 109 cells. Most of these newly formed cells were lost from the blood lymphocyte compartment within 3 wk. They are considered to be short-lived lymphocytes with half-lives of 3-8 days. About 90% of the blood lymphocytes were long-lived with a turnover time in excess of 1 yr in the blood lymphocyte compartment. The short-lived cells were both large and small lymphocytes, the long-lived cells were almost exclusively small lymphocytes. Lymphoid cell proliferation was virtually absent in the blood, very low in the marrow, and marked in the enlarged lymph nodes. The exchange of lymphocytes seemed to be partially intact between lymph nodes and blood but more or less completely blocked between marrow and blood.