S U M M A R YR factors transmissible to Escherichia coli from naturally occurring strains of Serratia marcescens belonged either to compatibility groups ( S and L) not represented amongst plasmids reported in other genera or to groups (C, FII, P and M) notable for their wide host range. 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 K 1 2 . The R factors were classified by compatibility in K 1 2 (Datta, 1974). Our purpose was to find out how much of the antibiotic resistance observed in
M E T H O D SSerratia marcescens. The sources of the 236 strains of S. marcescens studied are shown in Antibiotic resistance patterns were scored using a disc method (Hedges, 1974). Transfer of resistance to Escherichia coli ~1 2 .A rifampicin-resistant mutant of E. coli ~1 2 , 562-2 (Hedges, 1974) was used as recipient in tests for transfer. Methods were as described by Coetzee et al. (1972).