Nephrotoxicity is a major factor limiting the clinical utility of aminoglycoside antibiotics(AG). In this study, we have therefore investigated the usefulness of the renal tubular protein for a predictor of the nephrotoxicity of AG. We have also compared the nephrotoxicity among different AG. Among the simple fracture patients visiting orthopedics, for whom an AG seemed warrantable, we have sampled thirty-nine subjects--excluding those suffering from severe bleeding, taking drugs, or having renal disease which can affect the renal function. We have set three different groups by the following criteria; ten subjects in group I were given 250mg of Amikacin sulfate twice a day; fifteen subjects in GroupII were given 60mg of Micronomicin sulfate twice a day; and fourteen subjects in GroupIII were given 200mg of Isepacin sulfate twice a day. Urine from each patient was collected for 24 hours before, one week after, and two weeks after the drugs were given, and then the urinary concentrations of NAG, beta2-Microglobulin(beta2-MG), and electrolyte(Na+, K+, Cl-) were measured. The measurement of 24-hour urinary concentrations of NAG shows that, for all three groups, significant increase of the concentrations(P<0.01) is seen between the different times in the same group. The results of measurements of the 24-hour urinary concentrations of beta2-MG, and electrolyte(Na+, Ke+, Cl-) show their increase for all three groups but whithin the normal range. For the samples collected two weeks after the drugs were given, there is a significant decrease in the twenty-four-hour urinary concentrations of NAG(P<0.05) of Group III compared to Group I and II. The resulta of measurements of 24-urinary concentrations of beta2-MG, and electrolyte(Na+, K+, Cl-) show their increase for all three groups but whithin the normal range. In conclusion, we have seen that the nephrotoxicity of the AG appears for all three groups; but, when we compare the nephrotoxicity between the different antibiotics, the nephrotoxicity of Amikacin sulfate and that of Micronomicin sulfate appear stronger than that of Isepacin sulfate. Our data suggest the usefulness of sequential NAG measurements in monitoring and predicting aminoglycoside nephrotoxicity.