Feline leukemia virus (FeLV) infectious disease is one of feline infection diseases spreading broadly all over the world. For bedside diagnosis of FeLV infectious disease, an immuno-chromatographic assay was investigated. Five different monoclonal antibodies were developed against the major core protein FeLV-p27. Among them, the combination of FL6 and FL12, which had little epitopic overlap each other, showed the highest sensitivity with no cross-reaction to the other feline virus antigens when they were employed to the immuno-chromatographic assay. The system had a practical detection limit of 0.5 ng of FeLV-p27 per 0.1 ml of feline sera within 15 min. In comparison with clinical standard methods, the system gave rapidly and accurately the same diagnosis with neither false negative nor false positive. Moreover, it did not need any pretreatment of blood specimen.