Dosage of gentamicin based upon the weight of the patient (1.5–8 mg/kg per day) produced a wide range of concentrations of the drug in serum. In 10% of patients, the level is potentially toxic, but in 30% it is inadequate for therapy. Concentrations of gentamicin in urine are high, in cerebrospinal fluid low, and in bile, pleural, and pericardial fluid one-half to one-fourth that in the serum. Renal insufficiency requires an altered maintenance dosage. Hemodialysis clears gentamicin at a rate equal to 60% of that at which creatinine is cleared. Individual patterns in the response to dose were observed to be accelerated, typical, or dampened, giving brief high, average, or low levels of drug in serum. A significant inverse relation exists between the level of gentamicin in serum and hematocrit. Studies with radioactive gentamicin (100 µg/ml) showed that 10% was associated with erythrocytes, 30% with plasma proteins, and 60% in solution. The individual dose-responses emphasize the value of monitoring gentamicin levels in serum during treatment.