A specific polyclonal antiserum was prepared against a gel-purified 60 kDa extracellular protein of Listeria monocytogenes ATCC 19111 corresponding to protein p60 previously detected in culture broths of L. monocytogenes strains Mackaness and EGD [Kuhn, M. & Goebel, W. (1989), Infection and Immunity 57, 55-61]. Indirect immunogold labelling combined with transmission electron microscopy and high-resolution scanning electron microscopy were used to investigate the location and distribution of p60 on the bacterial cell surface. In bacteria grown to the early stationary phase about 25% of the extracellular protein was estimated to be associated with the cell surface. The anti-p60 antiserum proved to be Listeria-specific. In an indirect immunofluorescence test the antiserum reacted with Listeria strains representing all species and different serotypes, except L. seeligeri, L. welshimeri, L. grayi and L. murrayi. No immunological cross-reactions were observed with 27 strains of bacteria from 16 other genera. The value of the anti-p60 antiserum in developing a diagnostic assay for Listeria cells in environmental samples and foods is discussed.