Classification of strains of a bacterial species by noting their ability to produce colicines active against a battery of ‘indicator’ or ‘passive’ strains. In typing Shigella sonnei (the main application of the test) a grown shigella strain on agar is scraped off, the plate sterihzed by chloroform vapour, and re-seeded with 16 indicator strains of Esch. coli at right-angles to the stroke of the original culture. The pattern of inhibition of the varying EscK coli types indentifies the Shigella sonnei strain.