A derivative of pSC101, pLC709, was constructed by ligation of the HincII-A fragment of pSC101 to the mini-colEI plasmid pVH51 and to a DNA fragment encoding resistance to the antibiotics streptomycin and spectinomycin. Insertions of the transposon Tn1000 (γ-δ) into the pSC101 replication region of pLC709 were isolated following cotransfer of the plasmid with the sex factor F. The sites of insertion of the transposon were determined by restriction enzyme analysis and the replication and incompatibility properties of the insertion plasmids and DNA fragments cloned from them were analysed. The insertion mutations defined a locus, inc, of approximately 200 base-pairs that is responsible for pSC101-specific incompatibility. Two mutations adjacent to this region inactivate pSC101 replication but can be complemented in trans by a wild-type pSC101 plasmid, and thus define a trans-acting replication function, rep. The inc locus is within a larger region of some 450 base-pairs that is essential for pSC101 replication and that includes the origin of replication. This 450 base-pair segment can replicate in the presence of a helper plasmid that supplies the rep function in trans.