We report on a competitive, homogeneous immunoassay for the detection of the hapten digoxigenin. The assay is based on competitive fluorescence quenching by gold nanoparticles. Digoxigenin is indirectly labeled with the fluorophore Cy3B through bovine serum albumin and used as a marker. Gold nanoparticles functionalized with anti-digoxigenin antibodies serve as fluorescence quenchers. Free digoxigenin molecules in the analyte solution compete with the labeled markers for antibodies on the gold nanoparticles. The fluorescence signal depends linearly on the free digoxigenin concentration within a range of concentration from 0.5 to 3 ng mL(-1). The limit of detection is estimated as 0.2 ng mL(-1) and the limit of quantitation is estimated as 0.6 ng mL(-1). The method can be used to detect digoxin, a drug used to cure cardiac arrhythmia.