The plasma enzyme butyrylcholinesterase (BChE) is of clinical interest because of the occurrence of genetic variants with decreased ability to hydrolyse, and therefore inactivate, muscle-relaxant drugs such as suxamethonium. Analysis of BChE involves the determination of both enzyme activity and biochemical phenotypes which are used to determine the risk of so-called 'scoline apnoea'. Problems in analysis arise from both the lack of a universally accepted reference method and the variety of substrates and conditions employed for the determination of activity and phenotypes. Phenotype is determined by the use of specific enzyme inhibitors that produce phenotype-specific patterns of 'inhibitor numbers'. DNA analysis is now possible, and true genotypes can be obtained. The nomenclature in use for cholinesterase studies can cause problems in interpretation and reporting as there is poor understanding of the difference between phenotype and genotype, and terms are often, inappropriately, transposed. Techniques for both biochemical and molecular analysis of the enzyme are discussed.