An Escherichia coli mutant, HDF026, defective for growth of phage T4, has been characterized biochemically and genetically. The mutant displays an elevated level of degradation of abnormal proteins, such as puromycyl polypeptides or canavanine-containing polypeptides. Genetically, HDF026 appears to be an allele of rho, which also encodes the transcription termination factor and RNA-dependent ATPase, Rho. The mutation contransduces by phage PI with ilv, weakly suppresses polar mutations in gal, and permits some growth of lambda N- phage. Temperature sensitive lambda mutants in gene O exhibit a reduced efficiency of plating at intermediate temperature on HDF026 mutants; presumably the lambda Ots protein is rapidly degraded in these strains. The ability of wild-type lambda to grow on HDF026 is also reduced, apparently the result of the lambda N product deficiency. gal escape synthesis, which reflects the level of lambda N activity, is decreased 50-66% in the HDF026 mutant. lambda r32, which requires more N function than wild-type phage, does not grow at all in HDF026. A lon mutation, which decreases protein degradation, partially reverses some of these phenotypes, suggesting that they are related to the protein hyperlability of HDF026.