Cefazolin was studied bacteriologically and clinically with the following results. The minimal inhibitory concentrations of Cefazolin against 22 strains of clinically isolated E. coil were mostly distributed at 0.78 to 1.56 mcg/ml and were lower one to three tubes than those of Cephaloridine, Cephalexin, Ampicillin or Kanamycin in tube dilution method. Cefazolin was administered intramuscularly to 30 patients with various urinary tract infections. Clinical effects were satisfactory in 22 of the 23 patients with acute infections, though unfavorable in 7 patients with chronic infections. The total effectiveness rate was 73.3%. Slight pain at the injection site was complained in some patients. No abnormal findings were observed in blood picture, liver and renal function.