Ten individuals registered at the Danish Cholinesterase Research Unit were examined at the DNA level for the presence of the K allele of plasma cholinesterase, using amplification-created restriction sites (ACRSs). A further nine members of a family registered at the unit were tested for mutations of the K and atypical variants. The frequency of the K allele was calculated from examination of normal material from 25 individuals, representing 50 random alleles. The results show that the ACRS method successfully demonstrates the presence of the K variant, whose frequency in the Danish population was found to be 0.18. We conclude that this technique is a reliable and rapid non-radioactive diagnostic assay for detecting the plasma cholinesterase K variant.