In the present study, we describe the generation of a series of anti-interferon-alpha8 (IFN-alpha8)-specific monoclonal antibodies (mAbs), their characterization, and the establishment of a sandwich enzyme-linked immunosorbent assay (ELISA) system for human IFN-alpha8. The sandwich ELISA system is highly sensitive to human natural IFN-alpha8 (nIFN-alpha8), with a minimum detection limit of 50 pg/mL, which did not cross-react with the other IFN preparations and several cytokines tested. Using this ELISA system, pharmacokinetic properties of an IFN-alpha preparation administered in mice were examined. We found that IFN-alpha8 has higher vascular permeability and stability than IFN-alpha2 in the circulation. These results suggest that this ELISA would be very useful for determination of IFN-alpha8 protein concentrations in various experimental samples and also of pharmacokinetic properties of IFN-alpha preparations in human.