for this is not apparent. The response of lymphocytes to PHA stimulation in peripheral blood cultures was tested because of the active role of the lymphocyte in maintaining immunological:‘integrity. PHA is generally believed to evoke a non-specific blastogenic response in such cultures, since many more lymphocytes are transformed and proliferate with this agent than with specific antigens (such as tuberculin, tetanus, thyroid, pertussis, etc.). It is controversial whether or not PHA in this situation induces antibody formation. of the mitogen is unknown. The exact mechanism of action Conard and Demoise (1970), using autoradiographic and subcellular fractionation techniques, showed that a tritiated PHA was localized largely in the cytoplasm of transformed lymphocytes, with the greatest concentration in the mitochondrial fraction, which suggested that. such organelles may be involved in initiation of lymphocyte transformation. The Marshallese in this study showed a decreasing transformation of lymphocytes with PHA stimulation which was well correlated with increasing age. These results indicated that the percentage of lymphocytes that can respond to PHA was reduced as the Marshallese grew older. Such a finding appears to indicate a general decrease in immuno logical capacity of the lymphocytes, assuming that the response of these cells to specific antigens is similarly impaired. Peripheral blood counts showed that a decrease in lymphocytes was well correlated with aging and compatible with cellular depletion and reduction in immunological capacity generally noted in the aged (Ram, 1967). It would appear, therefore, from these results that with aging there is on an absolute basis a greate1 loss of PHA-committed lymphocytes than of non-committed types.The slight decrease not