Thymopentin is a five amino acid peptide fragment of thymopoietin that exhibits immunostimulatory and anticancer activities. Thymopentin decreases iNOS activity in the treatment of Sezary Syndrome, a severe form of lymphoma. In leukemia cells, thymopentin inhibits proliferation and colony formation and induces differentiation. In macrophages, thymopentin increases pro-inflammatory cytokine secretion, activation of NF-κB, production of HSP72, and signaling by JNK. Thymopentin shows clinical benefit when used as an adjuvant, improving the response rate in subjects administered hepatitis B vaccines.