Quinidine, an antiarrhythmic drug, was quantitated in serum by a commercially supplied enzyme immunoassay procedure (EMIT®). Replicate analyses of serum controls resulted in a within-assay coefficient of variation of ≤10% and a between-assay coefficient of variation of ≤7%. Patient quinidine results by enzyme immunoassay were compared to those obtained by a nonselective (protein precipitation) and a selective (solvent back-extraction) fluorometric method. Assay selectivity for quinidine versus other drugs and versus the 3-hydroxy metabolite of quinidine was determined. Clinical evaluation of the results indicates the enzyme immunoassay technique to be sensitive for quinidine and more selective than commonly used fluorometric methods relying on back-extraction or protein precipitation.