The production and characterization of a specific antibody for use in the radioimmunoassay of procainamide are described. Cross-reactivity was measured by a nonequilibrium competitive procedure. Procainamide analog concentrations resulting in 50% inhibition were: procainamide, 1.59 nmoles/ml; N-acetylprocainamide, 3.55 nmoles/ml; a propyl analog of procainamide, 398 nmoles/ml; procaine, 316 nmoles/ml; lidocaine, greater than 8000 nmoles/ml; and practolol, greater than 16,000 nmoles/ml. Variations in the ability to inhibit binding of labeled procainamide were related to structural similarities and differences. The affinity constant of the antibody for procainamide was 2.9 x 10(8) liters/mole as measured from a Scatchard plot. The assay allows the direct measurement of procainamide in a 0.1-ml aliquot of diluted serum. The advantages of this method over currently available techniques are its sensitivity, specificity, and simplicity. Furthermore, prior extraction of serum samples is not required. As little as 1 ng of drug/ml of serum can be detected by this method. The accuracy and precision were determined by adding known amounts of procainamide to human serum and then assaying five replicates of each concentration. The within-day and between-day coefficients of variation were 2 and 5%, respectively. The proposed method was used to determine the serum concentration after an intravenous dose of procainamide. A comparison of the radioimmunoassay results with values obtained by a GLC procedure showed excellent agreement.