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Biotyping, slime production, bacteriophage typing, serotyping, antibiograms, and plasmid profiles were used to characterize 19 Staphylococcus epidermidis strains isolated from 12 patients with prosthetic valve endocarditis and from 7 patients with native valve endocarditis. With the API Staph battery, 12 different biocodes with, at the most, three differences were obtained. Slime production was found for 10 strains (53%). Agglutinogens investigated by agglutination with two specific sera were found for 12 strains (63.1%). Three strains were phage typable (15.2%). Against a panel of nine antimicrobial agents, 15 different profiles were found. Multiply antibiotic-resistant strains were isolated from patients with prosthetic valve endocarditis when disease onset occurred less than 18 months after heart surgery and from patients with native valve endocarditis who received antibiotics immediately prior to their illness. All of the strains were available for plasmid analysis, and all the DNA profiles were distinct. On gels run in Tris-borate buffer, 73.7% of the strains had large plasmids of more than 30 megadaltons. A small plasmid of 2.8 megadaltons was found in multiply resistant strains and in strains resistant only to tetracyclines. None of the isolates appeared to be the same strain, and the bacteriological differences between the strains were confirmed mainly by the antibiotic susceptibility profile and the plasmid pattern analysis. These bacteriological results were in agreement with the clinical data.