This paper reports on the sensing of proteins using water-gated organic field-effect transistors. As a proof-of-concept, streptavidin and avidin were used, with a biotinylated polymer as the active sensing material. The latter is a copolythiophene modified to graft biotin by peptidic coupling. After characterization of its structure, it was integrated as the channel material into transistors and its interactions with several proteins were investigated. Non-specific interactions were reduced when the polymer surface was pretreated with 1-octanol. In this case, human serum albumin had no effect on the transistor characteristics whereas avidin and streptavidin led to a decrease of the drain current.