The therapeutic range of aminoglycoside antibiotics is relatively narrow; therefore short-term controls of serum concentrations can be recommended, particularly for high-dosage therapy and for renal insufficiency. For the aminoglycosides Amicacin, Gentamycin, Sisomicin and Tobramycin an enzymatic quick-determination test within 2 hrs is being described, which is based on the pH-deviation of a medium containing urea by hydrolytic activity of proteus mirabilis-bacteriae. In more than 180 measurements of patients' sera as well as of sera with known concentrations the method was tried out and compared with the conventional agar diffusion procedure. In strict compliance with the methodical directions, reproducable results could be achieved at any time within the range of1.0-30.0mug/ml; the number of faults in the quick-determination test was with +/- 8.4% about the same as in the agar diffusion test. In comparison with other microbiological or biochemical quick-procedures, the simplicity of the described method must be emphasized, thus it can be performed, too, in non-bacteriological laboratories.