Stable RNA of Escherichia coli was extensively degraded about 40 min after thermal induction of lysogenized lambda cI857 phages at 42 degrees C. When several nuclease-deficient host cells were tested, RNase I activity in the host cells was inferred to be involved in the RNA degradation. Ribosomal structure was detectably altered before the degradation of ribosomal RNA was observed. 30S and 50S subunits began to sediment at 25-28S and 45-58S, respectively, still containing intact RNA. Nonpermissive host cells lysogenized with lambda cI857 susR produced progeny phages in normal burst size after thermal induction and then degraded stable RNA, though they were not lysed. In contrast cells lysogenized with lambda cI857 susS produced ten times more progeny phages under the same condition, but did not degrade stable RNA. These results indicate that the lambda S gene product, which acts as a positive effector of lysis, induced the degradation of stable RNA, presumably by a still uncharacterized effect on the cytoplasmic membrane.