Procainamide and its major metabolite, N-acetylprocainamide, were measured by the homogeneous enzyme immunoassay technique (EMIT®). The reagents for the EMIT assays were supplied as a separate matched set for each assay. There is no crossreactivity by procainamide in the assay for N-acetylprocainamide or by N-acetylprocainamide in the assay for procainamide. Within-day precision determined by replicate analysis of samples in the therapeutic range gave a coefficient of variation of less than 5% for each assay. The day-to-day coefficient of variation was less than 6% for each assay. Quantitative results obtained by the enzyme immunoassay on serum samples from patients receiving procainamide were compared with the results obtained by a high pressure liquid chromatography procedure. For the procainamide assay the correlation coefficient (r) was 0.983; for the N-acetylprocainamide assay the correlation coefficient was 0.981. There were no false positives or false negatives. The immunoassay requires 50 μl of serum and the enzyme activity is measured in a spectrophotometer. An individual determination requires only 1 min to perform; therefore, the procedure can be used for either emergency or routine analysis.