In our previous studies, we found increased levels of urinary trypsin inhibitory activity in gentamicin-induced nephrotoxicity in rats. Following administration of the Bowman-Birk trypsin and chymotrypsin inhibitor (BBI), no proteinuria was detected in gentamicin-treated rats, and a decrease in creatinine clearance was noted in only 50% of the injected rats. In the present study, we examined the antimicrobial activity of gentamicin against Escherichia coli in the presence of BBI in gentamicin-induced nephrotoxicity in rats. We found that 50% of rats with E. coli-positive blood cultures died of septicemia. All the rats injected with E. coli plus gentamicin or E. coli plus gentamicin plus BBI survived, the latter showing no proteinuria or deterioration in creatinine clearance. In conclusion, BBI, which is an effective inhibitor of gentamicin-induced nephrotoxicity, does not affect the antimicrobial activity of gentamicin sulfate.