In order to elucidate the usefulness of cephacetrile (CEC), comparative trials with cefazolin (CEZ) were carried out in the patients with complicated infections of the urinary tract, and the following results were obtained: 1. There were no statistically significant differences or tendencies observed between the groups given CEC and CEZ in either of the global effects, bacteriological effects, rate of superinfection, relapse, relapse with bacterial alternation, drug usefulness, rate of improvements in symptoms and findings in all the cases. 2. According to a result of stratified analyses, there were statistically significant differences observed, showing more inferior results of CEC than CEZ in either stratum of "below 50" in the age, "acute" in the disease phase, "infections of the upper urinary tract" in the disease pattern, and "E. coli including mixed infections with gram-positive organisms" in the pathogenic strains. However, it should be taken into consideration that certain background factors influence greatly in the effects and there were some problems on deviated backgrounds in these strata. On the other hand, CEC displayed better bacteriological effects than those of CEZ in higher stratum of "chronic" in the disease phase and "Klebsiella, Proteus, Citrobacter, Enterobacter including mixed infections" in the pathogenic strains, and statistically significant differences were not observed. 3. In consequence of bacteriological studies, CEC showed stronger resistance than CEZ against beta-lactamase produced in all the strains of gram-negative bacilli. Particularly there were significant differences or tendencies observed in those of Proteus, Cirtobacter, and Enterobacter, and at the same time there were great differences in the activity of beta-lactamase between CEC and CEZ as the substrate. 4. In correlation of the MIC and effects, CEC showed weaker actions than CEZ against susceptible organisms, but slightly stronger actions against moderately or highly resistant strains. A study of the relationship between the MIC of pathogenic strains, beta-lactamase, and bacteriological effects, showed possibilities that even the high activity of beta-lactamase results in remarkable effects in susceptible strains and the drugs with stronger resistance against beta-lactamase given better results in moderately or highly resistant organisms. 5. The incidence rate of adverse reactions was 2 out of 51 cases in the CEC group (3.9%) and 3 out of 50 cases in the CEZ group (6.0%), showing no significant difference, and those symptoms were similar. In conclusion, when the usefulness of CEC in complicated infections of the urinary tract is compared with that of CEZ, the former is said to be equal to the latter without significant difference as a whole...