The amplification refractory mutation system (ARMS) is a simple, rapid and reliable method for the detection of any mutation involving single base changes or small deletions. We have applied ARMS methodology to the detection of mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Single ARMS tests have been developed for 11 CFTR mutations found in the northwest of England. ARMS reactions for the most common mutations have been multiplexed to give a test which will detect the presence of the delta F508, G551D, G542X, and 621 + 1G---T mutations in a DNA sample. The multiplex test has been validated by the analysis of over 500 previously genotyped samples and has been found to be completely accurate. The rapid detection of the most common mutations has enabled early molecular confirmation of suspected cystic fibrosis in neonates, rapid typing of cystic fibrosis patients and their relatives, and testing of sperm and egg donors.