Genetic recombination induced by structural damage in DNA molecules was investigated in E. coli K12 (lambda) lysogens infected with genetically marked phage lambda. Photoproducts were induced in the phage DNA before infection by exposing them either to 313 nm light in the presence of acetophenone or to 254 nm light. To test the role of the replication of the damaged phage DNA on the frequency of the induced recombination, both heteroimmune and homimmune crosses were performed. First, samples of a heteroimmune phage lambda imm434 P80 exposed to these treatments were allowed to infect cells lysogenic for prophage lambda cI857 P3. Phage DNA replication and maturation took place, and the resulting progeny phages were assayed for the frequency of P+ recombinants. Recombination was less frequent in infected cells exposed to visible light and in wild type cells able to perform excision repair than in excision-defective lysogens. Therefore, much of the induced recombination can be attributed to the pyrimidine dimers in the phage DNA, the only photoproducts known to be dissociated by photoreactivating enzyme. Second, in homoimmune crosses, samples of similarly treated homoimmune lambda P3 phages were allowed to infect lysogens carrying lambda cI857 P80. Replication of the phage DNA containing ultraviolet photoproducts was repressed by lambda immunity, and was further blocked by the lack of the P gene product needed for replication. The lysogens were purified and scored for both colony forming ability and for P+ recombinant prophages. The 254 nm photoproducts increased the frequency of recombination in these homimmune crosses, even though phage DNA replication was blocked. Irradiation with 313 nm light and acetophenone M, which produces dimers and unknown photoproducts, was not as effective per dimer as the 254 nm light. It is concluded from these results that certain unidentified 254 nm photoproducts can cause recombination even in the absence of DNA replication. They are not pyrimidine dimers, as they are not susceptible to excision repair or photoreactivation. In contrast, pyrimidine dimers appear to cause recombination only when the DNA containing them undergoes replication.