Conjugal mobility of ColE1 and related plasmids is promoted by a wide range of conjugative plasmids. ColE1 produces trans-acting products and has a region required in cis (bom ; basis of mobility) for such mobility. Here we show that plasmid pBR322 contains a functional bom sequence located within a 141 bp HhaI fragment. This bom region is functional for conjugation promoted by several different conjugative plasmids and is highly conserved in ColE1 and contains nic the putative origin of transfer. The orientation and position of bom with respect to the ColE1 vegetative origin of replication can be changed without affecting the frequency of conjugal mobility promoted by R64drd11.