In a study of 20 acutely ill elderly patients treated with cefotaxime (1 g, 2 X daily) the pharmacokinetics in serum and tissue fluid were examined. Patients with impaired renal function showed increased values for the area under the curve and half-life in both serum and tissue fluid. Patients with pathological peripheral circulation manifested delayed peak concentrations in tissue fluid. Although the passage of cefotaxime into tissue fluid was slow in the elderly, its concentration was higher than the minimal inhibitory concentration for most bacterial species of clinical importance and lasted for 5.5-7h in tissue fluid and for more than 10h in serum. Thus, this study clearly illustrates that the twice-daily dosage regimen used was quite adequate in elderly patients.