An enzyme-labeled immunometric assay has been developed for measuring digoxin concentrations in serum or plasma. Unitized, compartmentalized reagents are used with an automated sample-processing instrument. The enzyme activity of the processed sample, which is directly proportional to the digoxin concentration, is measured by using a reagent strip and the Ames Seralyzer reflectance photometer. The test takes less than 15 min, and digoxin concentrations are calculated from a two-point calibration line stored in the instrument. Within-run CVs for controls at four concentrations ranged from 2.3% to 3.8%; between-run CVs were from 1.5% to 2.6%. Results obtained with clinical serum samples correlated well (r greater than 0.96) with those obtained by fluorescent polarization immunoassay (Abbott TDx) and RIA (Clinical Assays and NML). This rapid and convenient method for monitoring digoxin concentrations in serum or plasma is particularly well suited for decentralized sites such as emergency rooms, urgent-care centers, and physicians' offices.