Immunoblot analysis of convalescent-phase sera of experimentally infected chimpanzees or monoclonal antibodies (MAbs) specific to the 90- and 40-kDa proteins of Mycoplasma pneumoniae indicated that both proteins were present in cytadsorbing, pathogenic strains PI-1428, M129, and FH but absent in noncytadsorbing, nonpathogenic strain M129-B176. Adsorption of convalescent-phase chimpanzee sera with virulent strain PI-1428 removed reactivity, whereas adsorption with avirulent strain M129-B176 did not remove reactivity to these two proteins. By using proteolysis and specific MAbs, we demonstrated that the 90- and 40-kDa proteins were surface exposed. Immunoelectron microscopy employing specific MAbs showed that the 90-kDa protein is localized on the terminal tip attachment apparatus. However, the MAb specific for the 40-kDa protein failed to indicate a similar localization. Nevertheless, these data, taken together, indicate that the immunodominant 90- and 40-kDa proteins are surface exposed, are localized on the terminal tip apparatus, and might be involved in the attachment mechanism.