A simple method, based on a modification of the amplification refractory mutation system (ARMS), for genotyping outbreak strains of measles and mumps viruses and detecting these in a simple enzyme immunoassay (EIA) is described. Fifty-three measles strains circulating at the time of an outbreak in London in 2000 and 26 strains circulating at the time of a mumps outbreak in Accrington, UK, in 1999 were investigated. All strains were genotyped by direct sequencing. ARMS primers were then designed to amplify the outbreak strain. The ARMS-EIA for measles and mumps detected all 36 measles outbreak strains as genotype D6, and all 15 mumps outbreak strains as genotype F, respectively. The sensitivity and specificity of both the measles D6 and Mumps F genotype ARMS EIA was 100% compared with direct sequencing. The results show that ARMS-EIA can be used as a rapid alternative to genotyping by direct sequence analysis in outbreak situations.