The study of bacterial plasmids has not always been as popular as it is today. For many years, the molecular biology of pro cary- otes was focused heavily on bacteriophage and plasmid investigations which were carried out in only a few laboratories. Whatever inter- est existed in plasmids concerned the role of these extrachromosomal elements in bacterial conjugation, genetic exchanges, and antibiotic resistance, as well as in the structure of plasmids themselves. Gradually, however, it became increasingly evident that many of the special characteristics displayed by bacteria of medical,...