An amperometric bilirubin biosensor was fabricated by complexing the Mn(II) ion with a conducting polymer and the final biosensor surface was coated with a thin polyethyleneimine (PEI) film containing an enzyme, ascorbate oxidase (AsOx). The complexation between poly-5,2'-5',2''-terthiophene-3-carboxylic acid (PolyTTCA) and Mn(II) through the formation of Mn-O bond was confirmed by XPS. The PolyTTCA-Mn(II) complex was also characterized using cyclic voltammetry. The PolyTTCA-Mn(II)/PEI-AsOx biosensor specifically detect bilirubin through the mediated electron transfer by the Mn(II) ion. To optimize the experimental condition, various experimental parameters such as pH, temperature, and applied potential were examined. A linear calibration plot for bilirubin was obtained between 0.1 microM and 50 microM with the detection limit of 40+/-3.8 nM. Interferences from other biological compounds, especially ascorbate and dopamine were efficiently minimized by coating the biosensor surface with PEI-AsOx. The bilirubin sensor exhibited good stability and fast response time (<5s). The applicability of this bilirubin sensor was tested in a human serum sample.