Antibodies to a protein present in the cell envelope of Hydrogenomonas facilis agglutinate many gram-negative bacteria and a few gram-positive bacteria. Immunodiffusion studies of extracts from the various organisms tested indicate the presence of components sharing antigenic determinants with the cell envelope protein in all bacteria which can be agglutinated and a few that cannot. Cross-reacting components were not detected in extracts of Mycoplasma laidlawii, Anacystis nidulans, and several eukaryotic organisms.