Monoclonal antibodies (MAbs) were raised against the purified fimbriae of Serratia marcescens US46, a strain expressing three morphologically distinct fimbriae. The widths of these fimbriae were 7, 4.5, and 3 nm, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified fimbriae showed three bands with molecular weights of 21,000, 20,000, and 19,000, respectively. This strain had mannose-resistant (MR) hemagglutinating activity and was agglutinated by yeast cells. Therefore, strain US46 appeared to have both MR and mannose-sensitive fimbriae. In the immunoblot analysis, all MAbs reacted with the 20,000-molecular-weight subunit when given a choice of three differently sized subunits. Immunoelectron microscopy showed these MAbs attached to the MR fimbriae with the largest width (7 nm). The antigenic cross-reactivity of fimbriae was examined by an MAb-mediated agglutination test. All MR strains of S. marcescens and some mannose-sensitive strains were agglutinated by the MAbs. The serological homogeneity of MR fimbriae was confirmed by a spot test, using the crude purified fimbriae from several MR strains of S. marcescens. In other gram-negative rods, clinical isolates of Klebsiella spp. with hemagglutinating activity were agglutinated, but clinical isolates of Escherichia coli and Enterobacter spp. were not.