We report the isolation of LexA mutant proteins with impaired repressor function. These mutant proteins were obtained by transforming a LexA-deficient recA-lacZ indicator strain with a randomly mutagenized plasmid harbouring the lexA gene and subsequent selection on MacConkey-lactose indicator plates. A total of 24 different lexA(Def) missense mutations were identified. All except three mutant proteins are produced in near-normal amounts suggesting that they are fairly resistant to intracellular proteases. All lexA(Def) missense mutations are situated within the first 67 amino acids of the amino-terminal DNA binding domain. The properties of an intragenic deletion mutant suggest that the part of the amino-terminal domain important for DNA recognition or domain folding should extent at least to amino acids 69 or 70. A recent 2D-NMR study (Lamerichs et al. 1989) has identified three alpha helices in the DNA binding domain of LexA. The relative orientation of two of them (helices 2 and 3) is reminiscent of, but not identical to, the canonical helix-turn-helix motif suggesting nevertheless that helix 3 might be involved in DNA recognition. The distribution of the lexA(Def) missense mutations along the first 67 amino-terminal amino acids indeed shows some clustering within helix 3, since 8 out of the 24 different missense mutations are found in this helix. However one mutation in front of helix 1 and five mutations between amino acids 61 and 67 suggest that elements other than helices 2 and 3 may be important for DNA binding.