Nine temperature-sensitive and defective mutants of phage lambda deficient in lysis of the host cell have been analyzed in respect to location of the site of mutation and phenotypic properties. Seven of the isolates are mutant in cistron R and produce either a thermolabile endolysin or no detectable enzyme. The remaining two mutants, one temperature-sensitive and one defective, produce high levels of apparently normal enzyme. These mutants denote the involvement of at least one phage cistron other than R in the lytic process.