During thiopentone‐fentanyl‐nitrous oxide anaesthesia and using a cumulative design, we studied the dose‐response relationship of mivacurium in 8 patients: 7 patients phenotypically homozygous for the atypical plasma cholinesterase gene and 1 patient homozygous for the silent gene. The estimated mean ED₅₀ and ED₉₅ were 15 and 20 μg. kg. bw^‐1 in patients homozygous for the atypical gene, and 13 and 16 μg. kg. bw^‐1 in the patient homozygous for the silent gene, respectively. The results indicate that mivacurium is 4‐5 times more potent in patients homozygous for the atypical or the silent gene than in patients with normal plasma cholinesterase activity and phenotype.