A previous survey of enterococci had identified five strains of Streptococcus faecalis (K55 and SB94) - two subspecies liquefaciens (K60 and K88) and one zymogenes (K87) - and two S. faecium strains (K46 and SB69) which were resistant to tetracycline and streptomycin but susceptible to gentamicin. All the S. faecalis strains and K46 were in addition resistant to erythromycin but only the S. faecium strains were penicillin and ampicillin resistant. The minimal inhibitory concentrations of a further six antibiotics were determined. These values confirmed that in S. faecalis strains, erythromycin resistance was accompanied by resistance to lincomycin and pristinamycin IA, a phenotype typical of macrolide - lincosamide - streptogramin B - type (MLS) antibiotics resistance. The erythromycin resistant K46 however, although resistant to lincomycin, was pristinamycin susceptible and so the basis of resistance is unknown. S. faecalis K60, K87 and SB94 were resistant to kanamycin and neomycin as was S. faecium K46 but all strains were susceptible to spectinomycin. The phenotypes were consistent with resistance mediated by enzymic modification of streptomycin with adenyltransferase (6) and of kanamycin and neomycin with phosphotransferase (3') (5")-III. Erythromycin and tetracycline resistances were expressed constitutively in all strains. Only one S. faecalis (K88) was found to be chloramphenicol resistant and as is typical of Gram-positive bacteria, resistance was inducible. The ability to produce bacteriocin was restricted to beta-haemolytic strain K87 and to strain SB94. Subsequent results indicated that strain K87 probably produced more than one bacteriocin, the activity of which was repressed in the parental strain but which, in derivatives, could be enhanced by the presence of streptomycin. Evidence for the location of resistance, haemolysin and bacteriocin genes was sought from study of the transfer characteristics and stability of markers and from examination of the plasmid content of parental strains and their derivatives. The well characterised S. faecalis subspecies zymogenes strain DS5 (Clewell et al., 1982b) was included for comparison in transfer and curing experiments. All the S. faecalis strains aggregated in response to a cell free filtrate of a plasmid free recipient strain JH2-1, indicating the presence of at least one conjugative plasmid although the low transfer frequencies of most resistance genes in broth matings suggested that response was not necessarily encoded by antibiotic resistance plasmids. Transfer of beta-haemolytic activity and all resistance markers was observed after broth matings but the range of transfer frequencies between strains was wide. Furthermore, the incidence of transfer could be variable particularly in the transfer of DS5 erythromycin resistance and all K87 antibiotic resistances which seemed to be dependent on the production of active donor bacteriocin. Matings of S. faecalis strains carried out on membrane filters were only marginally more efficient in terms of transfer frequencies but were superior with regard to reproducibility of transfer. No antibiotic resistance transfer from S. faecium donors was observed after broth matings and only SB69 tetracycline resistance transferred after filter mating at very low frequency. Several resistance determinants and those encoding β-haemolysin were found to be capable of retransfer indicative of association with genes specifying conjugative ability. Analysis of transconjugant phenotypes revealed that the tetracycline resistance gene of K55, the streptomycin resistance gene of K88 and β-haemolytic activities were always transferred alone but some resistance markers were usually co-transferred with other donor markers.