The mature human IL-10 is a protein with 160 amino acids and the functional IL-10 is a homodimer. IL-10 is expressed by a variety of cells, including mouse T cells (TH1, TH2, and Tr1 subsets), B cells derived from peripheral blood, tonsils or spleen, Epstein–Barr virus (EBV)-transformed B cell lines, Burkitt’s lymphoma, AIDS lymphomas, monocytes, placental trophoblasts, bronchial epithelial cells, and certain tumor cells including melanomas and carcinomas of various origin. IL-10 plays an important role in inflammatory and immune responses. The biological activities of IL-10 include both immunosuppressive and immuno-stimulatory effects. For example, IL-10 can suppress the production of pro-inflammatory cytokines by monocytes and neutrophils, and down regulates the expression of activating and co-stimulatory molecules on monocytes and dendritic cells. IL-10 also can improve the growth of B cells and mast cells, and inhibit or enhance the activities of T cells depending on their activation conditions. The different polymorphisms in the IL-10 gene promoter have been demonstrated the association with both SIDS (sudden infant death syndrome) and sudden unexpected death due to infection. Based on the functions of IL-10, it has been suggested that an IL-10 antagonist can be applied to boost anti-viral immunity against viruses such as EBV and that the IL-10 molecule itself may be used as an anti-inflammatory reagent. This IL-10 ELISA is a ready-to-use 3.5-hour solid phase immunoassay capable of measuring IL-10 levels in cell culture supernatant, serum, plasma, and other biological fluids in the range of 0 to 1600 pg/mL. This assay has shown no cross-reactivity with other cytokines tested.