Rules for the prediction of protein conformation (Chou, P. Y., and Fasman, G. D. (1974), Biochemistry 13, 211, 222) have been applied to the N-terminal sequence 1-60 of the lac repressor. This analysis predicts beta structure at sequences 4-9 and 15-20, helices at 26-32, 38-45, and 52-57, and beta turns at 48-51 and 14-17. Repressor mutants lacking operator binding capacity in which Pro replaces Ser-16 and Ala replaces Thr-19 (Weber, K., Platt, T., Ganem, D., and Miller, J. H. (1972), Proc. Natl. Acad. Sci. U.S.A. 69, 3624) have no effect on the prediction of beta structure at residues 15 to 20, which suggests that the polar side chains of Ser-16, Tyr-17, Gln-18, and Thr-19 participate in intermolecular hydrogen bonding with conplementary polar groups on the lac operator. The loss of operator binding capacity on replacement of Ala by Val at position 53 in the repressor results from a predicted secondary structural change from helix to beta structure for residues 52-57 which can be transmitted to the N-terminal sequence via a beta turn at residues 48-51. The basic residues at positions 33, 35, and 37 between the helical regions 26-32 and 38-45 probably bind to the phosphate groups on the operator on complexation. It is proposed that complex formation involves the interaction of either a beta structure (residues 15-20) or a right-hand twisted antiparallel beta-pleated sheet (residues 4-9 and 12-20) with operator DNA.