A biosensor system using flow injection analysis (FIA) has been developed for the analysis of glucose in human serum. The system consists of the enzyme glucose oxidase incorporated into graphite paste modified with the electroactive material tetracyanoquinodimethane (TCNQ). TCNQ acts as an efficient mediator for oxidation of the reduced enzyme at (200 \mathrm{mV} \mathrm{vs} \mathrm{Ag}) / AgCl. The flow injection assay described has detection limits of (2 \mathrm{~mm}) glucose using a (100-\mu 1) sample injection through a (250-\mu) l sample loop. Data are presented to show the effect of sample injection volume and flow rate on the response of the FIA sensor. The biosensor exhibited excellent reproducibility for 800 injections. The loss of response after 800 injections was due to leaching of TCNQ from the graphite paste. Each assay takes 3 min giving a sample throughput of 20 per hour at a flow rate of (30 \mathrm{ml} / \mathrm{h}). The sensor was applied to the determination of glucose in human serum. The glucose measurements are in good agreement with those of a commercially available spectrophotometric method. Data showing the effect of interfering substances, ascorbic acid and acetaminophen, on the response of the sensor are also reported. 1993 Academic Press, Inc.