Recombination between lacZ alleles in crosses of lambdalacZ(1) (-) x lambdalacZ(2) (-) and F(-)lacZ(1) (-) x lambdalacZ(2) (-) in Escherichia coli (lambda) can be stimulated manyfold by UV irradiation of one of the lambdalacZ phages [Porter, R. D., McLaughlin, T. & Low, B. (1979) Cold Spring Harbor Symp. Quant. Biol. 43, 1043-1048]. Analogous stimulation has now been observed by coinfection of the cells by UV-irradiated lambda phage which carries no lac region. This indirect stimulation is not dependent on induction of the SOS system. The bacterial uvr system can effectively remove the damages on the lambda DNA which cause the indirect stimulation. Among a number of mutations tested, only ssb-1 was found to cause a drastic decrease in the indirect stimulation. Indirect stimulation was caused only by using phage that had a region of homology with the recombining phage. The homologous region can be separated from the recombining region by an extended nonhomologous region (>7.9 x 10(3) base pairs). This implies that damages to the DNA molecule, which stimulate recombination, can be located very far from the recombining region of the molecule.