An immuno-displacement assay technique utilizing disposable sensing elements and based on fluorescence detection of rhodamine labeled immuno-complexes has been developed. Enkephalin has been used as a model analyte. The disposable sensing element is a strip coated with a thin gold layer by vacuum evaporation. The gold surface is modified by immobilized enkephalin. The enkephalin layer is treated with a rhodamine labeled immuno-complex against enkephalin to form a fluorescent layer. A presence of free enkephalin in the media causes a displacement of the labeled immuno-complex. This results in a decrease in surface fluorescence, which is proportional to the concentration of the free enkephalin (analyte). A detection range 1-100 μM for the enkephalin assay has been observed for a 10 min assay time.