The value of plasmid profile determination as an epidemiological tool in Pseudomonas aeruginosa infections was investigated by determining the prevalence of plasmids in 450 Pseudomonas aeruginosa strains and comparing the technique with other epidemiological tools. Since only 13.9% of these strains harbored plasmids and the majority of these plasmids were antibiotic resistant, the technique appeared to be less appropriate as an epidemiological tool in this organism than other techniques. Comparison of results obtained from plasmid profile determinations with those from antibiotyping, serotyping and pyocin typing in 50 non-epidemic strains showed the technique gave highly reproducible results and was sensitive; its ability to discriminate could be improved by additionally performing conjugation assays and hydrolysis of plasmidic DNA with restriction enzymes. It is concluded that plasmid profiles provide important epidemiological information on Pseudomonas aeruginosa infections when performed in conjunction with either serotyping or, more importantly, pyocin typing.