Bacterial host cells of different rec genotypes were used to investigate genetic requirements of ultraviolet light (UV)-induced homologous plasmid-chromosome recombination. Plasmid DNAs which contained a wt or mutant lacY gene were irradiated with UV prior to transformation into Escherichia coli host strains which contained the complementary lacY allele. Surviving transformants were screened to determine the directions of UV-induced recombinational exchange between the bacterial and plasmid lacY genes, by assaying lactose utilization. Nonreciprocal chromosome-to-plasmid recombination was 100% dependent on the recA gene and greater than 80% dependent on the recF gene, but not dependent upon the recB gene of E. coli. In contrast, reciprocal plasmid-chromosome recombination was strictly dependent on the recA gene, greatly dependent (approx. 80%) on the recF gene, and moderately dependent on the recB gene. Nonreciprocal plasmid-to-chromosome recombination was only induced at very low frequencies, and appeared to be moderately dependent on the recB gene, but not dependent on the recF gene. UV-induced plasmid-chromosome recombination appeared to proceed by a two-step mechanism. In this model, the initial step is recF-dependent, recB-independent, and either resolves to become a nonreciprocal chromosome-to-plasmid recombinant, or proceeds to the second step. The second step is moderately recB-dependent and results in the reciprocal exchange of plasmid-chromosome sequences.