Organisms common as urinary pathogens were grown in urine and their effect on the urinary pH was followed. Proteus organisms caused a rapid rise in the pH to high levels. When glucose was present in the urine, a slight initial rise was superseeded by a fall of the pH. In mixed infections with P. vulgaris and E. coli the pH did not rise when E. coli was dominating but rose at equal concentrations of the two species and when Proteus dominated. These results offer possible explanations why the pH does not always rise in the urine in patients with urinary tract infections with Proteus. Possible alternative mechanisms are under investigation.