In recent years, Serratia marcescens has become established in certain localities as an agent of hospital infection and cross-infection (Clayton & von Graevenitz, 1966; Wilfert, Barrett & Kass, 1968; Davis, Foltz & Blakemore, 1970; Wilkowske, Washington, Martin & Ritts, 1970). In general, strains of S. marcescens isolated from infective lesions differ from those from other sources in being non-pigmented and antibiotic resistant (Ewing, Johnson & David, 1962; Clayton & von Graevenitz, 1966). Medeiros & O'Brien (1969) and Schaefler et al. (1971) described strains of S. marcescens, isolated from hospital patients, which were able to transfer R factors to Escherichia coli. Transfer of resistance to E. coli has also been reported from strains of S. marcescens isolated in France (Grimont & Dulong de Rosnay, 1972; Scavizzi, 1972; Lemosquet-Villemon, Morel & Freymuth, 1973)9 We have collected strains of S. marcescens, most, but not all, clinical isolates, from widely separate geographical areas; each strain was tested for antibiotic resistance and for R factors transmissible to E. coli K129 The R factors were classified by compatibility in K12 (Datta, 1974). Our purpose was to find out how much of the antibiotic resistance observed in S. marcescens is characteristic of that genus and to what extent it is shared with other bacterial genera (Coetzee, Datta & Hedges, 1972; Datta & Hedges, 1972a; Hedges, 1974). The R factors described by Medeiros & O'Brien (1969) and Lemosquet-Villemon et al. (1973) were included in this study.