Monoclonal antibody WM-25 inhibited the in vitro growth of 13 F38 isolates from goats with contagious caprine pleuropneumonia but not 7 heterologous mycoplasma isolates representing four different species. In contrast to results with polyclonal antisera, growth inhibition by monoclonal antibody WM-25 was specific for F38 mycoplasma isolates and constituted a reliable means of distinguishing F38 from other mycoplasmas.