Using the 5th fraction of thymosin, it was possible to enhance the capacities of peripheral blood lymphocytes to form rosettes with sheep erythrocytes and to respond to PHA in those cases of cancer or sarcoidosis in which these functions were depressed. Moreover, the 5th thymosin fraction increased the resistance of CBA mice to infection with virulent H37Rv mycobacteria and augmented their ability to acquire delayed hypersensitivity.