We have designed a P1 vector (pCYPAC-1) for the introduction of recombinant DNA into E. coli using electroporation procedures. The new cloning system, P1-derived artificial chromosomes (PACs), was used to establish an initial 15,000 clone library with an average insert size of 130-150 kilobase pairs (kb). No chimaerism has been observed in 34 clones, by fluorescence in situ hybridization. Similarly, no insert instability has been observed after extended culturing, for 20 clones. We conclude that the PAC cloning system will be useful in the mapping and detailed analysis of complex genomes.