Of the 26 urinary E. coli strains employed in the present study, 16 were haemolytic and 10 were non haemolytic in character. Of the 16 haemolytic strains injected through the exposed femoral vein of the rats, 12 showed the lesions of pyelitis, pyelonephritis and proliferation of glomerular tuft while 2 showed only the lesions of pyelitis and pyelonephritis. It was interesting to note that none of the nonhaemolytic strains produced any lesion in the animal. The data demonstrated a positive correlationship between haemolytic activity of E. coli and pyelonephritis in rats.