Guinea pigs, in which cells with long life span were selectively labeled (3H-thymidine), were joined in parabiosis to nonlabeled syngeneic litter mates at a time when label reutilization detectable by radioautography could be excluded. The distribution of labeled cells was investigated quantitatively using radioautography and liquid scintillation counting in the marrow and blood at the time of establishment of parabiosis and again at its termination 2 wk later, when the thoracic duct lymph, lymph nodes, spleen, and thymus were also examined. Single animals labeled in the same manner served as controls. Of all cells with a slow rate of turnover and long life span, only small lymphocytes entered the circulation and crossed the anastomosis in detectable numbers. As indicated by the similar percentages of labeling in respective tissues, a complete intermixing of long-lived lymphocytes occurred in the bone marrow, lymph, lymph nodes, and spleen of the parabionts. The sum of the per cent labeled lymphocytes in two parabionts was in agreement with the extent of labeling in respective tissues of single controls. The presence of a minor population of lymphocytes with a long life span was confirmed in the marrow. Ten to 30 times as many labeled long-lived lymphocytes migrated into the bone marrow of initially unlabeled animals as were found in an equal volume of blood. The majority, if not all long-lived lymphocytes migrate to the marrow from the blood, and they also reenter the blood. They have a similar life span and in parabionts equilibrate in a similar manner as recirculating long-lived lymphocytes.