The splenic lymphoid tissue of mice reared in a conventional environment regressed with increasing age. Immunogloblin G positive plasma cells gathered along the peniciUi in the senile spleen as well as in the adult control spleen. The number of plasma cells around the peniciUi showed no difference in the senile spleen as compared to the adult spleen. Since mature plasma cells remain in the lymph node for a short time only and seldom divide there, our results indicate that the transformation from B lymphocyte to plasma cell is not affected by age.