Mutants of phage T2, of the group calledrII mutants, are known to lack the parent phage's ability to form plaques on strains ofEscherichia colilysogenic for phage λ That they are capable of initiating desoxyribonucleic acid synthesis in the infected lysogenic host cells but are incapable of maintaining it has been shown by the present study using T2r7and -E.coliC112 (λs). In contrast with the superficially similar behavior shown by some other phages (e.g. λv1h), ultraviolet irradiation and consequent induction of the lysogenic culture do not release the prophage-linked inhibition of T2r7DNA synthesis. Net ribose-nucleic acid synthesis is stopped in all cultures on infection with T2r7.