The capacity of Thymomodulin, a thymic derivative, to restore the antibody response and the number of circulating leucocytes in immunodepressed animals was studied. Administered daily to cyclophosphamide-treated mice, Thymomodulin significantly stimulated the recovery of the antibody response to sheep erythrocytes after a second administration of the antigen. The white blood cell count also showed a rapid and significant increase. Antibody titres were enhanced to normal range values again in immunodepressed rabbits. The possible mechanism of these findings is discussed. Some preliminary results obtained in restoring the immune response in aged mice are also reported.