A new prototype direct reading glucose electrode working with glucose oxidase and hydrogen peroxide was preliminarily tested clinically during insulin-induced hypoglycemia in eight healthy subjects, and during hyperglycemia in five dysregulated diabetic patients. The results for 282 whole blood samples were compared to those of our routine method, which measures the glucose concentration in whole blood. The correlation was: y = 1.05.x - 0.05 mmol/L, r = 0.99. The glucose electrode measured a glucose concentration of 10.5 mmol/L +- 0.49 mmol/L (between-day imprecision) in a control serum (glucose 10.0 mmol/L). The glucose electrode supposedly responds to the activity of glucose that equals the molality (mmol glucose per kg water). The ratio of results with the glucose electrode and our routine method was lower than the expected ratio between water concentration in calibrator and whole blood, which is 1.18. A steep gradient from blood sample to glucose electrode, depending on the diffusion coefficient and hematocrit might explain the discrepancy.