The nucleotide sequences involved in the illegitimate recombination of four recombinants between bacteriophage lambda DNA and pBR322 in E. coli (lambda TA6, lambda KA3, lambda TA1R, and lambda KA7) were determined. Each resulted from recombination between regions of homology of 10 to 13 base pairs. The presence of a recA+ allele was found to stimulate recombination between lambda DNA and pBR322 approximately 10-fold. Lambda TA6, lambda KA3, and lambda KA7 were isolated in the presence of a recA+ allele and therefore, may have been generated by the recA recombination system. However, lambda TA1R was isolated in a recA mutant, and was presumably generated by a different recombination system. The possibility that it was generated by DNA gyrase is discussed. Two recombination events were required to form lambda KA7, which may indicate that it also was generated by DNA gyrase.