Abstract 1. 1. The minicircular DNA of Escherichia coli 15 can be isolated as a DNA-protein complex (form I★) by sucrose gradient centrifugation in low salt (0.1 M NaCl). 2. 2. Form I★ can be converted into the non-complexed supercoiled form I in high salt (1 M NaCl) or by incubation with Pronase. It differs from the DNA-protein complexes of other plasmids by not undergoing a conversion to the open circular form II under conditions known to affect protein structure. 3. 3. Synthesis of the protein component of form I★ is inhibited by chloramphenicol. After a prolonged treatment with chloramphenicol the replication of the minicircular DNA continues but the newly replicated DNA is found almost exclusively in form I.