A recombination site in the transducing bacteriophage lambda plac5 DNA has been structurally elucidated. Comparison of primary structures of E. coli lac-operon (distal end of lacZ gene, Z-Y spacer, and proximal end of lacY gene) described earlier with corresponding segments of bacteriophages lambda CI857 and lambda plac 5-2 DNAs sequenced in this paper showed that the bacterial DNA insert ends immediately after Z-Y spacer, just before the initiating triplet ATG of lacY gene. It thus follows that in contrast to the earlier conception, the insert does not seem to include any part of lacY gene. The recombination sites in both phage and bacterial DNA contain structurally homological segments about 20 b. p. long (crossover region), with two extra basepairs in the bacterial DNA (AT in the sense-strand). We suppose that the very dinucleotide plays a substantial role in initiation of recombinational event: causing formation of a nonperfect heteroduplex structure, it determines the T-A internucleotide bond to be endonucleolytically cut (crossover point) followed by exonucleolytic elimination of the extra links (AT) and reciprocal strand exchange. The second recombination site in lambda plac5 DNA has been localized by us within lacI gene as being close to the HindII site (nucleotides 854 to 859 of the gene). The structures of the two regions of site-specific recombination may shed light upon mechanisms of the phage abnormal excision leading to formation of transducing phages.