We have isolated a derivative of plasmid R100.1, following integration into and excision from the Escherichia coli chromosome, which exhibits the ability to produce circular copies of its resistance-determinant (r-det) region. The plasmid also shows unusually frequent loss of the r-det resistance genes. Both these phenomena are inheritable plasmid characteristics and are dependent on the host recombination system. The production of r-det can be repressed in trans by the presence in the same cell of a second plasmid closely related to R100.1.