We have demonstrated the feasibility of quantitatively detecting selected toxic materials through their inhibitory effect on an enzyme electrode that utilizes colloidal gold-immobilized horseradish peroxidase and does not require a mediator. Quantitative detection of azide, cyanide, thiourea, sulfide, and dichromate is demonstrated. The sensitivity and inhibition kinetics for this immobilized enzyme electrode are found to be different from those observed previously for homogeneous horseradish peroxidase. Possible reasons for this difference are discussed. Due to the availability of a large number of enzymes and their toxic inhibitors, this work based on immobilized enzyme inhibition coupled to an electrode surface significantly broadens the possible applications of biosensors and offers alternative methods for toxic substance determination.