Abstract We have isolated mutants of T5 whose growth is inhibited in wild-type lambda lysogens, and which we refer to as “lambda-repressed” or “ lr” mutants. These mutants grow normally in nonlysogenic strains, as well as in strains in which the lambda gene rex is defective or deleted. The results of recombination and complementation tests indicate that lr mutations occur in gene I of the FST-DNA. Amber mutants on the left side, but not on the right side, of the lr-1 locus are also partially inhibited in Su + lysogens, suggesting that lr mutants can only be found in part of gene I. Evidence is presented that the lr gene I product is synthesized in lambda lysogens, and is partially active. The possible mechanisms of this growth inhibition are discussed.