The technique of performing the Dienes test is described, as well as its application to the investigation of an outbreak of cross-infection with Proteus mirabilis. The test was found to possess a high degree of specificity. The frequency of reactions of identity among 48 proteus isolates obtained from unrelated sources and tested against themselves was 1 in 70, based on a total number of 1128 tests. These 48 isolates could be grouped into 35 “Dienes strains”