There have been reports of toxic reactions to ester-type local anesthetics in patients with genetic anomalies of plasma cholinesterase in so-called dibucaine-resistant homozygotes or heterozygotes. Because of these reports, 23 patients with prolonged apnea after administration of succinylcholine were studied with Kalow's method for abnormal dibucaine number (DN), which characterizes this condition, and for the history of local anesthetic toxicity. Thirty-eight relatives of abnormal individuals were studied in the same manner. Two atypical dibucaine-resistant homozygotes and the relative of one of these had toxic reactions to ester-type local anesthetics. Although low doses of ester-type local anesthetics may be tolerated by atypical homozygotes, these agents are best avoided and amide-type local anesthetics or tetracaine, which is hydrolyzed at a normal rate, should be used. Individuals with atypical plasma cholinesterase should wear a Medic-Alert bracelet or amulet so that complications and fatalities can be avoided.