Thymalfasin (thymosin-alpha 1) is an immunomodulating agent able to enhance the Thl immune response. It has been evaluated for its immunomodulatory activities and related therapeutic potential in several diseases, including chronic hepatitis B and C, AIDS, primary immunodeficiency diseases, depressed response to vaccination and cancer. The basis for effectiveness in these conditions is primarily through modulation of immunological responsiveness, as thymalfasin has been shown to have beneficial effects on numerous immune system parameters and to increase T-cell differentiation and maturation. Thymalfasin is responsible for reconstitution of immune function when thymic tissue is given back to thymectomized animals. In addition, thymalfasin has been shown to have efficacy in multiple experimental models of immune dysfunction, mainly, infectious diseases such as hepatitis (woodchuck) and influenza (mouse), and cancer such as melanoma (mouse) and colorectal carcinoma (rat) where thymalfasin has shown antitumor effects.