Plasmid D, a hybrid of plasmids P-lac and R1 drd19, mediates polarized chromosome mobilization from one origin in Proteus mirabilis strain PM5006, while the parental plasmids neither individually nor combined mobilize this chromosome. To elucidate its acquired mobilizing ability plasmid D was characterized physically in relation to P-lac and R1 drd19. Restriction patterns of these plasmids were compared and it was shown that D consists of P-lac and only the r-determinant (r-det) of R1 drd19. A mechanism for the formation of plasmid D, via transduction of the r-det and subsequent transposon-like integration into P-lac, involving insertion sequence IS1, was suggested. Evidence for aberration in plasmid D DNA as a result of r-det integration into P-lac was attributed to IS1 elements which flank the r-det. Recombination regions of parental plasmid DNA were located on HindIII fragments alpha and beta of plasmid D and were subsequently inserted in vitro into IncP-1 plasmid RP4 that fails to mobilize the P. mirabilis chromosome. RP4::HindIII alpha plasmids did not mobilize the latter chromosome, but rendered the Proteus host lac+. RP4::HindIII beta plasmids pMC1 and pMC17, containing the fragment in opposite orientations, mobilized the P. mirabilis chromsome. For pMC17, mobilization was indistinguishable from that of plasmid D, i.e. having the same orientation and the same single origin. However, mobilization promoted by pMC1 was from two distinctly different origins, different from that of pMC17. This apparently deviates from known examples where inversion of homologous DNA inserted into plasmids leads to mobilization from the same origin but in reverse direction.