To simplify the selection of tests for bacteriological typing methods, such as bacteriophage, bacteriocin, and biotyping, a computerised method was assessed. This uses a numerical index of discrimination (D) to facilitate the selection of an efficient typing set. The computer programs take the most discriminatory test as the initial test in the partial typing set, and then select the next test by combining each of the remaining candidates with the partial set and choosing the test which maximises D. This cycle is repeated until the remaining candidates do not increase the discriminatory power of the typing set. Options are provided for the investigator to pre-select certain tests for inclusion or exclusion from the typing set. It is concluded that the numerical index D is a simple means of test selection, but it must be emphasised that it is important to combine its use with data on the incidence of reaction in each test, on reproducibility, and on the similarity among tests.