In studying molecular mechanisms of specialised transduction. the lac1 (E. coli)-Ea47(λ) DNA junction in transducing bacteriophage λplac5 has been structural1y elucidated, thus yielding the complete sequence of λplac5 DNA including the lac5 substitution, a well-known segment of lambdoid vectors. The λphac5 DNA is shown to consist of 19368 bp (λ left arm) + 3924 bp λplac5 substitution)+25353 bp (λ right arm), totally amounting to 48645 bp. The presence of the phage pbL promoter near to the right end of the lac5 insert is shown. The lacl gene distal end in λplac5 proved to be much longer than it was oostulated earlier. codinq for 224 C-terminal amino acid residues of lac repressor. Both the recombination studied in this paper and the earlier studied abnormal prophage excision (2,3) occur near to Chi-like structures (chi*Lac1 and chi*lom, respectively). On thebasis of the data obtained, a key role of the E. coli RecBCD system and Chi-like sequences in the formation of deletions in bacterial cells is suggested. © 1988 IRL Press Ltd.