HAPA-B was administered intramuscularly to 20 patients with complicated urinary tract infections to evaluate its clinical effects and safety in conformity with the estimations of efficacy of antimicrobial agents standardized by the UTI Committee. The overall clinical efficacy was excellent in 5 patients, moderate in 8 patients and poor in 7 patients. The effectiveness rate was 65.0%. Overall clinical efficacy as classified by type of infection was 66.7% in 12 patients with a single infection, and 62.5% in 8 patients with mixed infection. Bacteriological studies showed that 29 of the 34 strains isolated from the urine of the 20 patients were completely eradicated, and that the bacteriological response was 85.3%. The following side effects were detected in both blood chemistry checks and clinical symptoms. Peripheral blood examinations showed an increase in eosinophil level in one patient. Kidney function tests revealed a slight elevation of s-creatinine in this patient, too. In another patient, eruptions developed on his body on the 2nd day, but it was not necessary to discontinue the administration of HAPA-B.