Determination of the dibucaine number of serum permits the differentiation of genetically normal serum cholinesterase from the dibucaine-resistant variant. This communication presents a method for the rapid determination of dibucaine numbers on an automated analyzer that is in widespread use. With this method, dibucaine numbers for seventy-eight normal, four heterozygous, and five dibucaine-resistant sera were 79 +/- 3,60 +/- 10, and 20 +/- 3 (+/- SD), respectively. There was good agreement between the original method and the automated method with a correlation coefficient of 0.98 on split sample comparison of thirty-eight normal, four heterozygous, and five abnormal sera. Advantages of this method over the original include the lack of need for advance preparation and that it can provide results within one-half hour of phlebotomy.