Using a radioimmunoassay for tobramycin, the drug could be detected in serum and urine for 11 days after end of tobramycin therapy in a patient with unimpaired renal function. The concentration curves in serum and urine were roughly parallel. In a semilogarithmic graph, the tobramycin disappearance curve in serum exhibited at least three phases, with estimated half-lives of 0.3, 3.9and 169 h, respectively. Kinetics of the terminal phase appeared to be of the first order. The results were similar to those recently obtained for gentamicin. The possible inadequacy of the two compartment open model, hitherto used for describing the pharmacokinetics of gentamicin and tobramycin, is discussed.