Hemagglutinin titers produced by CY-treated mice given SRBC and 2 x 10⁷ normal syngeneic spleen cells were always much higher than those found when the spleen cells were obtained from animals previously given ALS. Titers could be raised to normal levels if the animals in this latter group received additional injections containing mixtures of normal syngeneic thymus and marrow cells. Marrow cells alone were completely ineffective, while inocula which only contained thymus cells were much less effective than mixtures of thymus and marrow cells. These results suggest that immunosuppression by ALS is associated with the inactivation of both thymus and marrow-derived lymphocytes.

In other experiments CY-treated mice received SRBC and mixtures of thymus and marrow cells from both untreated and ALS-treated donors. No decrease in the immunological competence of cells located within the thymus of ALS-treated donors was demonstrable in these experiments. Marrow cells were slightly affected but to a markedly lesser degree than were spleen cells of ALS-treated animals. In a final experiment, it was possible to show that the thymus and marrow cells of ALS-treated animals could repair the immunological defects which were present in their own spleen cell populations.