A rapid and simple plasmid isolation procedure was developed for the epidemiological analysis of plasmid-mediated antimicrobial resistance. By this method, plasmid DNAs ranging in molecular weight between 2.0 and 122 X 10(6) could be detected. Various bacteria, such as strains of the family Enterobacteriaceae, Pseudomonas aeruginosa, Haemophilus influenzae, and Staphylococcus aureus, could be analyzed. The plasmid DNA obtained could be directly used for restriction endonuclease analysis without further purification. In addition, this method made it possible to analyze several cultures at the same time.