Plasmid-mediated resistance to chloramphenicol (Chlr), erythromycin (Eryr), tetracycline (Tetr) and clindamycin (Clindr) was transferred from three clinical isolates of Bacteroides fragilis and one faecal isolate of B. thetaiotaomicron to strains of B. fragilis, B. distasonis and Escherichia coli, and subsequently to B. fragilis and E. coli second-and third-stage recipients in series. Successful transfer was achieved by membrane-filter and centrifugation techniques that provide stable cell-to-cell contact but not by simple mixed broth culture. Chlr Eryr Tetr and Clindr Eryr were transferred at high frequency (1.9 x 10(-3)-1.8 x 10(-4)) but Tetr was transferred at low frequencies (1-1.6 x 10(-6)). Segregation of resistance markers was observed with selection for Tetr when donors were Chlr Eryr Tetr and Chlr Tetr. All transcipients were identical with the parent recipient strains but had the resistance markers of the donor strains. Resistance to antibiotics other than tetracycline was cured by growth with subinhibitory concentrations of aminoacridines and ethidium bromide for 24 h; cure of solitary Tetr required longer incubation (21 days). Identical plasmid DNA bands were demonstrated by agarose-gel electrophoresis in all the donor and corresponding transcipient strains but plasmids were not found in the recipient strains or in strains cured of resistance. Plasmid-mediated transferable antiobiotic resistance in Bacteroides spp. may compromise the treatment of infections and may provide a reservoir of antibiotic resistance in the intestinal flora.