Standardized methods are essential if phage typing of staphylococci is to be reliable and if the results obtained in different laboratories are to be compared. This paper, prepared on behalf of the Subcommittee on Phage Typing of Staphylococcus of the Nomenclature Committee of the International Association of Microbiological Societies, gives a detailed account of methods that have been found satisfactory for propagating the phages and defines a standard testing routine by which the stability of the phage preparations can be verified. Technical details are given of methods recommended for determining the phage type of staphylococci and the interpretation of differences between the types is discussed. The need for close collaboration between the epidemiologist and the laboratory worker in the use of phage typing is emphasized.