The biosensors based on the enzyme electrode in a flow injection analysis suffer an interference from hemacyte, macromolecules and some lower molecular weight species in whole blood, plasma, serum or urine, besides endogenous electroactive species such as ascorbate. This often results in an increasingly serious tailing phenomenon in the response current peak of substrates. A unique method used daily is to cover the outer enzyme membrane with a porous ultrafilter film to isolate it from hemacyte and macromolecules but not from some smaller molecules. In the present paper, we first introduce a tailing inhibitor-ion pair reagent into the flowing buffer system for eliminating the tailing phenomenon from the complex matrices on biosensors. When 0.25 or 0.5% tetrabutylammonium chloride (V/V) was added to 0.02 M phosphate buffer, serious lags of whole blood, plasma, serum and urine on the two biosensors for uric acid and glucose disappeared entirely so that their response time was shortened to about 50 seconds; their base lines also became more smooth and stable. Thus, the resulting two biosensors were capable of detecting over 60 biosamples per hour. The reported experiment shows that the tailing inhibitor has effects on the response current and response time of various biosamples and on the concentration of the dissolved oxygen in buffer, with no evident effect on the recoveries of glucose and urate in biosamples with both sensors. The finding confirmed that some lower molecular weight species with negative charge result in the serious tailing phenomenon in the response current peaks of various biosamples on the glucose and urate biosensors in a flow injection system.