Mice of the AKR strain develop T-lymphocyte leukemia originating in the thymus. We found increased numbers of B-lymphocytes and B-lymphocyte-colony forming cells (CFCBL) in the thymus of older AKR mice. No age-related variation of CFCBL numbers was observed in spleen or lymph nodes, nor in lymphoid organs from two other inbred strains of mice. Fifty to ninety percent of the lymphocytes from thymus and spleen colonies were immunoglobulin-positive. CFCBL of normal thymus were more sensitive to cortisol and irradiation in vitro than CFCBL of spleen and lymph nodes. 3 X 10(-8) M cortisol resulted in a 50% reduction of the number of thymic-CFCBL while the corresponding concentration for lymph node and splenic-CFCBL was 10(-7) M. D0 for thymic-CFCBL was 45 rad, for spleen and lymph nodes 54 rad. CFCBL from thymus and spleen were not killed by hydroxyurea treatment indicating that they were not in the S phase of the cell cycle. Velocity separation showed that CFCBL of young and aged non-leukemic thymus sediment at the rate of 2.5-3 mm/h. In the thymomas, however, the sedimentation coefficient ws 2-8 mm/h indicating that both small and large CFCBL were present.