It is shown that the amino acid sequence and the DNA gene sequence of the 25 amino-terminal residues of the lac repressor protein of Escherichia coli are homologous with the sequences of five DNA-binding proteins: the cro repressor proteins from phage lambda and phage 434, the cI and cII proteins from phage lambda, and the repressor protein from Salmonella phage P22. The region of homology between lac repressor and the other proteins coincides with the principal DNA-binding region of cro repressor. In particular, residues Tyr-17 through Gln-26 of lac repressor correspond to the alpha-helix Gln-27 through Ala-36 of cro repressor, which we have postulated to bind within the major groove of the DNA and to be primarily responsible for the recognition of the DNA operator region by the protein [Anderson, W. F., Ohlendorf, D. H., Takeda, Y. & Matthews, B. W. (1981) Nature (London) 290, 754--758]. By analogy with cro repressor, we propose that residues 17--26 of lac repressor are alpha-helical and that this helix and a twofold-related alpha-helix in an adjacent subunit bind within successive major grooves of the lac operator, which is in a right-handed Watson--Crick B-DNA conformation. Also, by analogy with cro repressor, we suggest that residues Thr-5 through Ala-13 of lac repressor form a second alpha-helix and contribute, in part, to DNA binding. The proposed structure for the DNA-binding region of lac repressor is consistent with chemical protection data and with genetic experiments identifying the probable locations of a number of the residues of the repressor protein that either do or do not participate in DNA binding.