A recombinant plasmid containing the Drosophila melanogaster P element transposon was microinjected into mouse zygotes. Dot-blot analysis indicated that one of the newborns contained a single copy of the microinjected DNA per haploid mouse genome equivalent; two other newborns had integrated multiple copies of the P element construct. Southern mapping revealed that the entire plasmid, including both pBR322 sequences and P element sequences, had integrated in each of the three animals. In the two mice carrying multiple copies of the microinjected DNA, the copies appear to be linked in a tandem head-to-tail array. Therefore, in each of the three newborns integration of P element sequences has occurred by a mechanism which is distinct from that observed when the same plasmid is injected into Drosophila embryos. Analysis of DNA from the offspring of one of the transgenic mice showed no indication of transposition of P element sequences.