THE principle of resistotyping, i.e., the differentiation of bacterial strains by the degree of their resistance to selected chemical substances, was first described by Elek and Higney in 1970. Their method of resistotyping strains of Escherichia coli from cases of urinary-tract infection gave results that were in good agreement with those of serotyping. A similar system was worked out for Shigella sonnei (Elek, Davis and Miles, 1973) and the resistotypes agreed well with the colicine types and with the epidemiological findings in various outbreaks. The present work is an attempt to assess the value of resistotyping of Staphylococcus aureus, for which there is already a long- established epidemiological tool in the form of phage typing. MATERIALS AND METHODS