The influence of teicoplanin and aminophylline (theophylline ethylenediamine) on their mutual steady-state pharmacokinetics was studied in two parts, namely experimental and clinical studies. The concentrations of teicoplanin and aminophylline (theophylline) were evaluated at several times after intravenous administration of teicoplanin (3 and 15 mg/kg) and aminophylline (5 mg/kg) in 12 rabbits. Ten COPD patients were treated for 5 consecutive days with a short-term intravenous infusion of 240 mg aminophylline twice daily; ten more patients received for 5 days a short-term intravenous infusion of 200 mg teicoplanin twice daily. On the last day, blood samples were taken for teicoplanin and theophylline determination by means of the HPLC method. Subsequently, while aminophylline and teicoplanin were continued at the same dosage, the first ten patients received in addition a short-term intravenous infusion of 200 mg teicoplanin twice daily, and the second ten patients a short-term intravenous infusion of 240 mg aminophylline twice daily. After 5 days the serial assays of serum teicoplanin and theophylline were repeated. Our results demonstrated, in both experimental and clinical studies, no influence of theophylline on the steady-state pharmacokinetics of teicoplanin. Likewise teicoplanin had no significant effect on the steady-state pharmacokinetics of intravenous administration of theophylline in the form of aminophylline.