Abstract: A specific binding assay method and reagent system based on the use of an inhibitory anti-enzyme, e.g., antibody or fragment thereof, as the label component. Such method and reagent system have been improved by selection of anti-(glucose-6-phosphate dehydrogenase) [anti-G6PDH] as the anti-enzyme label. The improved label is monitored by its ability to inhibit G6PDH. The resulting assay is more sensitive, requires lesser quantities of reagents, is less susceptible to sample interferences, and employs a reagent system having greater stability than the published prior art method employing an anti-peroxidase label. The present invention is particularly applicable to homogeneous immunoassays for determining substances appearing at low concentrations in biological fluids such as urine and serum.