The selectivity and sensitivity of glucose measurements at carbon-paste based amperometric biosensors are greatly enhanced through the use of ruthenium-dispersed graphite particles. The improved performance is attributed to the substantial lowering of the overvoltage for the reduction of the hydrogen peroxide product. Hence, cathodic measurements of glucose can be caried out at an optimal potential range (-0.15 to +0.20 V). Contributions from easily oxidizable substances (e.g. acetaminophen, ascorbic and uric acids) are eliminated, without the need for mediators or membrane barriers. The electrocatalytic action of the ruthenium sites results also in a substantially improved sensitivity. A fast flow injection operation is illustrated.