Characteristics of transferable resistance of 72 entero-bacteriaceae resistant to one more antibiotic, isolated from February to September 1972, showed that this was common. Multi-resistant strains permit one to obtain transfer of various characteristics either alone or together in 76 per cent of cases, whereas strains with less than three characteristics, transferred in only 9 per cent of cases. The transfers were uncommon in the case of Proteus compared with other enterobacteriaceae and, in particular, Klebsiella, Escherichia coli and Salmonella.