Twenty-five strains of Providencia alcalifaciens from various countries have been characterized by one-dimensional SDS-PAGE of cellular proteins. They comprised 15 from human faeces, one from duck faeces, one from a guinea-pig eye and eight from unknown sources. Also included, for reference purposes, were the type strains of three other Providencia species. The protein patterns, which contained 45-50 discrete bands, were highly reproducible and were used as the basis for two numerical analyses. In the first, in which the principal protein bands (in the 33-40 kD range) were excluded, the 25 Prov. alcalifaciens strains formed, at the 83% S level, a single cluster whilst the three Providencia reference strains remained unclustered. In the second, which included all the protein bands, the 25 Prov. alcalifaciens strains formed 10 clusters at the 85% S level. We conclude that high resolution PAGE combined with computerized analysis of protein patterns provides the basis for typing clinical strains of Prov. alcalifaciens. Reference strains of each of the 10 PAGE types identified are available from NCTC for inclusion in future studies.