The structures of two plasmids, one found in one of the transformants with putative products of an in vitro DNA rearrangement reaction containing the Tn3 transposase, and another found as a spontaneous tnpA negative derivative of an overproducer of the Tn3 transposase, have been elucidated. They both had shown non-conventional results in restriction enzyme analysis. For the determination of the structures, we used restriction enzyme analysis, denaturation and renaturation experiments, and DNA nucleotide sequencing. The structures turned out to be 'staggered' head-to-head dimers of the original monomer plasmids, containing gigantic inverted repeats separated by two identical spacers which are also in inverted orientations themselves. Two alternative models for the mode of origin of such a structure, a bimolecular model and a unimolecular one, are discussed. The circumstances in which these two plasmids occurred suggest possible involvement of the Tn3 transposase in their generation.