A two-compartment pharmacokinetic model was used to characterize serum concentrations and to predict tissue accumulation of gentamicin in 47 treated patients. Postmortem tissues were obtained in six cases; in each instance, tissues yielded the predicted amount of drug. Slow release of tissue-bound gentamicin accounts for its prolonged retention in the body. The two-compartment model adequately predicts gentamicin accumulation from serum concentrations and explains why this antibiotic persists in serum and urine. (JAMA 238:327-329, 1977)