SUMMARY1. Twenty‐five cases of infection of the urinary tract were treated with terramycin; there was a marked difference between the clinical and bacteriological results.2. Clinical cures numbered ten, whilst fourteen cases improved, and only one was regarded as a failure.3. In only two of the cases were the infecting bacteria eliminated; in the remainder the original infecting species remained or replacement species were isolated.4. The organisms isolated before the start of treatment were sensitive to terramycin; all the organisms present after treatment were more or less resistant to the antibiotic.5. The urinary concentration of terramycin was always many times thein vitroconcentration required to inhibit the growth of the organisms concerned.6. The significance of these results is discussed and certain conclusions are drawn.We are indebted to Professor T. J. Mackie, Professor L. S. P. Davidson, and Mr David Band for much helpful criticism. Messrs Chas. Pfizer and Co. kindly supplied the terramyci