A lambda gt11 clone, designated M1 and having a 0.8 kilobase (kb) insert, was selected by screening a Mycoplasma gallisepticum (M.g.) genomic DNA library with antisera against M.g. cells and their membrane proteins. The sequence of a 1.7 kb EcoRI fragment of genomic DNA covering the entire M1 insert revealed a long open reading frame, TM-1, that encoded a polypeptide with a deduced molecular weight of 29 kDa. An antiserum raised in chicken against the TM-1 polypeptide, which was produced by recombinant Escherichia coli cells and purified by column chromatography, inhibited growth of M.g. cells in vitro. Moreover, chickens immunized with this polypeptide were partially protected from a challenge with virulent M.g. This polypeptide may serve as the basis for a vaccine against M.g. infection.