Methyl alpha-D-mannopyranoside (alpha MM), a competitor inhibitor of the binding of mannose by Escherichia coli, was tested for its ability to prevent infection of the urinary tract of mice with infective strains of the organisms. Injection of the bacteria in the presence of the drug resulted in a considerable reduction in the number of bacteriuric mice. In this system alpha MM was inactive against Proteus mirabilis in accordance with its inability to inhibit the adherence of this organism to epithelial cells in vitro, and methyl alpha-D-glucopyranoside proved inactive against both E. coli and P. mirabilis.