Interleukin-10 (IL-10) is produced predominantly by T-helper lymphocytes and macrophages. IL-10 inhibits lymphocyte-induced cytokine production (CSIF--cytokine synthesis inhibitory factor), cooperates in B lymphocytes activation as well as macrophages--monocytes deactivation (MDF--macrophage deactivating factor). Due to its immunosuppressive properties IL-10 may be used in therapy of some T-cell induced diseases.