Ascending urinary tract infections with Proteus were induced in rabbits by intravesical inoculation of bacteria. The urinary findings of leucocytes, bacteria, alterations in the urinary pH and the morphological lesions in the urinary tract were studied. In spite of heavy growth of bacteria in urine and in the kidneys and inflammatory lesions in the urinary tract there were remarkably low concentrations of leucocytes in the urine at pH values above 7.75. This was the case in most of the rabbits infected with P. vulgaris. When the urinary pH was below 7.75, there was a considerable concentration of leucocytes. This was observed in some of the rabbits infected with P. vulgaris and in most of those infected with E. coli. The demonstration of leucocytes in urinary sediments as a criterium of urinary tract infection must be reconsidered, especially in infections due to proteus bacteria as there is evidence that they cause leucocyte disintegration in vivo.