This work is a first step in the development of a specific probe for the study of the distribution and colonization of leaf surfaces by pink-pigmented, facultatively methylotrophic (PPFM) bacteria of the genus Methylobacterium. A polyclonal antiserum was produced in rabbits against whole cells of PPFM strain PC1, isolated from surfaces of white clover leaves. Major heat labile antigens were found in extracts of sonicated cells using the Ouchterlony double diffusion method. Very small amounts of a heat stable antigen were also observed. The major antigens were found in extracts of each of fifteen PPFM strains tested but were not found in extracts of other clover heterotrophs nor in extracts of other methylotrophs tested. The distribution of antigens in ultrathin sections of PPFM cells was investigated using PC1 antisera and gold labelled protein A. Gold particles were seen mainly in the outermost layer of the homologous strain, but isolated and washed cell envelopes of strain PC1 like other strains retained very little antigen. Sections of other PPFM strains showed the major antigens were located mainly in the cytoplasm.