Limitin is an interferon (IFN)-like cytokine that we recently identified and cloned on the basis of its ability to arrest the growth or kill lympho-hematopoietic cells. This 182 amino acid protein has approximately 30% sequence identity with IFN-alpha, IFN-beta, and IFN-omega. Limitin binds to the IFN-alpha/beta receptors and induces IFN regulatory factor-1, thereby indicating that limitin constitutes a new prototype of the type I IFN family. As with previously known IFNs, limitin inhibited B lymphopoiesis in vivo as well as in vitro. In addition, limitin not only modified the proliferation and function of peripheral T lymphocytes, natural killer cells, and bone marrow stromal cells but also had antiviral activity. Therefore, limitin is a multifunctional cytokine with several potential cellular targets. Because to date we have found no influence of limitin on normal myeloid and erythroid progenitors, limitin is unique among the IFNs. Type I IFN family contains IFN-alpha, IFN-beta, IFN-omega, and IFN-tau, and IFN-alpha is composed of at least 14 subtypes. All IFNs have anti-proliferative, immunomodulatory, and antiviral effects and influence to each other in the body. Limitin should play a role in the complex IFN network, and its human homologue would be useful as a therapeutic agent if it lacked myelosuppressive activity.