A method for the continuous, in vivo measurement of glucose concentration in animal body fluids such as blood, comprises implanting in the body, in registry with the system for the body fluid, a glucose monitor adapted to measure the concentration of glucose as a function of changes in such electrical charge. The glucose monitor includes an electrode with an electrical charge-transfer medium comprising a reversible complex of a binding macromolecular component and a charge-bearing carbohydrate component. The electrode includes a selectively permeable membrane permitting the ingress and egress of the body fluid. In operation, glucose present in the body fluid displaces the charge-bearing carbohydrate, which then participates in the electrical activity of the electrode so that a measurable change in electrical activity can be observed when the electrode is connected to an appropriate current-responsive meter. Preferably, the macromolecular component comprises one or more lectins, and the charge-bearing carbohydrate component may be one or more of the sugars for which testing is desired.