Recent studies in animal models have demonstrated that in contrast with humans, cephalothin (CTIN) does not increase gentamicin (GENT) nephrotoxicity, but rather protects against it, particularly when CTIN is given simultaneously with GENT. To investigate this phenomenon in humans a study was designed in which 67 patients suffering from mild infections were investigated. Thirty-three of them served as the control group receiving GENT alone at a dose of 1.5 mg/kg/8 hourly, while the remaining 34 received CTIN at a dose of 2 g or 3 g 8 hourly by i.v. bolus, either simultaneously with GENT or separated by a 4-h interval. Findings showed that: (a) cylindruria developed in 66.6% and 82.3% and 82.3% in the GENT and GENT + CTIN groups respectively, (b) urinary beta-glycuronidase activity increased in 57.5% and 75% (c) serum creatinine exceeded by 0.3 mg the initial values in 21.2% and 27.6% and (d) the blood urea was above 50 mg% in 18.1% and 17.6% of the patients. These results indicate that: (a) regardless of the route and order of administration simultaneous treatment did not protect against nephrotoxicity in humans; (b) the combination of GENTA plus CTIN has no synergistic effect on the production of elevated serum creatinine and rising blood urea; (c) urinary beta-glycuronidase is not a significant predictor of eventual nephrotoxicity; (d) the following risk factors influenced the appearance of nephrotoxicity in both groups: (1) elevated GENT trough levels greater than or equal to 2 mg/l; (2) a course of treatment longer than 10 days.