The labelling of peripheral blood lymphocytes of the mouse with [3H]thymidine.

The labelling of lymphocytes in the peripheral blood of mice following a single injection of [3H]thymidine was studied in autoradiographs exposed for 185 days before processing. The proportion of labelled cells was found to be greater than could be accounted for by new lymphocyte production. In addition, stimulation by sheep red cells 2 hours after giving [3H]thymidine resulted in the appearance of more labelled lymphocytes in the blood, but the greatest differences from control mice occurred relatively late in the immune response after cell proliferation in the responding lymphoid organ had reverted to control levels. The results are discussed on the basis that they indicate synthesis of DNA and reutilization of label by lymphocytes, which then circulate in the blood, these processes possibly occurring in the bone marrow and lymphoid tissues.