We have chemically synthesized oligopeptides corresponding to the NH2-terminal stretch of two gene products, designated FimG and FimH, of the fim gene cluster of Escherichia coli. These synthetic peptides, designated S-T1FimG(1-16) and S-T1FimH(1-25)C, evoked antibodies in rabbits that reacted with 14- and 29-kilodalton subunits, respectively, of dissociated fimbriae encoded by the recombinant plasmid pSH2 carrying the genetic information for the synthesis and expression of functional type 1 fimbriae. Neither of these fimbrial proteins was detected in dissociated fimbrial preparations from nonadhesive E. coli cells carrying the mutant plasmid pUT2002, containing a restriction site-specific deletion of fimG and fimH. Anti-S-T1FimH(1-25)C inhibited the adherence of type 1 fimbriated E. coli to epithelial cells. Immunoelectron microscopy revealed that anti-S-T1FimH(1-25)C, but not anti-S-T1FimG(1-16), bound to intact type 1 fimbriae of E. coli at the fimbrial tips and at long intervals along the fimbrial filaments. Anti-S-T1FimG(1-16) appeared to be directed at epitopes not accessible on the intact fimbriae and consequently failed to bind to intact fimbriae or to block fimbrial attachment. Our results suggest that the fimG and fimH gene products are components of type 1 fimbriae and that FimH may be the tip adhesin mediating the binding of type 1 fimbriated E. coli to D-mannose residues on mucosal surfaces.