Small GTPases of the Rab family are key regulators of membrane trafficking. Monoclonal antibodies are useful tools for identifying proteins that interact with other proteins and for examining their tissue distribution. We selected a monoclonal antibody against Rab8 of Bombyx mori L. It specifically recognized amino acid residues 30-109, which are conserved among Rab8 proteins, and did not recognize any other Rab proteins. Western blotting using the antibody revealed one band in the brains of B. mori and rat. Far-Western blotting analysis detected three proteins interacting with Rab8. These results indicate that this antibody is useful for clarifying the physiological function of Rab8 of B. mori and other species. This is a report of a study on a monoclonal antibody against insect Rab protein.