In this paper, we report on the isolation and sequence analysis of mutations that confer an induction-deficient phenotype to lambda repressor. A total of 16 different mutations, which occur at 13 different sites in the repressor gene, have been characterized. For most of the mutant lysogens, frequencies of spontaneous induction in a recA+ strain were reduced dramatically in comparison with those for a wild-type phage, and these mutant lysogens showed little or no prophage induction after UV irradiation. The immunity properties of cells containing the mutant repressors show that all of the mutants but one exhibit operator-binding properties indistinguishable from wild-type repressor.