A novel potentiometric enzyme immunoassay technique, utilizing polystyrene beads In conjunction with a gas sensing membrane electrode, is described. The technique is illustrated with the measurement of digoxin via competitive inhibition of anti-digoxin-horseradish peroxidase conjugate activity. The use of the gas sensing electrode permits polystyrene beads to be used without concern for turbidity In the measurement system. The method is convenient, inexpensive, and yields sensitivities In the nanogram range.