We investigated the effects of low levels of sulfamethoxazole and trimethoprim on biosynthesis, expression at the cell surface, and hemagglutinating activity of type 1 fimbriae from a urinary tract isolate of Escherichia coli. The mannose-sensitive hemagglutination of E. coli was reduced after growth in either of the antimicrobial agents. Moreover, trimethoprim affected antigenic fimbrial expression at the bacterial cell surface. Fimbrial subunit synthesis, as detected in lysates of solubilized whole bacteria, was inhibited in bacteria grown with one-half the minimum inhibitory concentration (MIC) or trimethoprim. Organisms grown in low doses of trimethoprim and sulfamethoxazole (one-thirty-second the MIC each) together exhibited marked reduction of fimbrial synthesis, expression, and hemagglutinating activity. Neither agent induced any detectable effect when used alone at the same concentrations. These results demonstrate that the synergistic activity of these drugs at concentrations below the MIC influence the synthesis, expression, and adhesive properties of type 1 fimbriae.