A competitive radioimmunoassay for the measurement of digoxin in a serum sample in which the nonradioactive digoxin competes with a constant amount of a radioactively labeled digoxin tyramine analog comprising 3-O-(4-hydroxy-phenethylcarbomoyl) digoxigenin for binding cites on a limited amount of digoxin antibody. The percentage of the radioactively labeled digoxin which is bound to the antibody is inversely proportional to the concentration of digoxin in the serum sample. The concentration of digoxin in the serum is determined by the comparison with standards containing measured amounts of unlabeled digoxin.