SUMMARY A previous study described the isolation of multiply-resistant strains of Escherichia coli from the gastro-intestinal tract after the ingestion of a strain carrying an R factor (Rl) that confers resistance to ampicillin (A), chloram-phenicol (C), kanamycin (K), streptomycin (S) and sulphadimidine (Su). One such strain carried the R factor R174 (ACKSSuT)-which was in addition resistant to tetracycline (T)-and another carried the factor R157 (AST). The minimum inhibitory concentrations of these antibiotics for E. coli strains, isogenic but for the R factors, were consistent with the formation of factor R174 by recombination of the AST and ACKSSu resistance determinants of the other two R factors. The factor Rl 74 determined a streptomycin-phosphorylase enzyme also present in factor R157, together with the strepto-mycin-spectinomycin resistance determinant of factor Rl, and all three R-factors shared the same type of ampicillin resistance. Factor R174 carried all the other antibiotic-resistance determinants (TCKSu) of the parent plasmids and is thus probably a recombinent of factor Rl and R157. DNA-DNA hybridisation showed that factor R174 contained almost all of the DNA-base sequences of factors R157 and Rl. There was also approximately 50% homology between the latter two R factors which may account for the occurrence of recombination.