A study of the electrochemical behaviour of ascorbic acid (AA) at the powder microelectrode revealed that exhaustive electrolysis of AA could be achieved within a narrow range of potential. As a result, the interference of AA in the amperometric detection of bioactive molecules in body fluid samples can be practically eliminated by employing the powder microelectrode technique. Elimination of the interference of AA can be further improved if the monitored biomolecules are strongly adsorbed on the surface of carbon powder. A good example of such a bioactive compound is uric acid (UA), which can be strongly adsorbed on the surface of acetylene black powder. In a mixture containing 0.8 muM-0.4 mM UA and a large excess of AA in 0.1 M phosphate buffer solution, the separation between the potentials of oxidation current peaks of UA and AA reaches 400 mV, and the height of the UA peak is linear with respect to the UA concentration. A very simple procedure for the detection of UA in human serum and urine samples has been worked out to illustrate the above-stated principle. Application of the same principle in the amperometric detection (and probably simultaneous determination) of other electroactive biomolecules also seems promising. (C) 2000 Elsevier Science B.V. All rights reserved.