Twenty-six Serratia marcescens strains belonging to fifteen different biotypes were found to produce bacteriocins active against Escherichia coli. On the basis of the pattern of bacteriocin sensitivity of E. coli mutant strains, immunological assays and Southern blot hybridization with a probe for the S. marcescens bss (bacteriocin 28b structural) gene, it was concluded that all these strains apparently produce chromosomally encoded bacteriocins related to bacteriocin 28b. To confirm this conclusion, the genes encoding the bacteriocin produced by one of these strains (S. marcescens JF246) were cloned in plasmid pBR328. E. coli harbouring recombinant plasmid pDG301 produced a bacteriocin active against E. coli and immunologically related to bacteriocin 28b. Immunoblotting experiments showed that bacteriocins 28b and L appear to have the same apparent molecular mass (45 kDa). Furthermore, recombinant plasmid pDG301 DNA hybridized with a bss gene probe.