Cell wall lytic activity was found in particles of the lipid-containing bacteriophage ø6. The activity can be extracted from the virion with Triton X-100 in the presence of salt. This treatment removes the membrane-like envelope of the virion which includes five proteins. The lysin requires detergent for in vitro activity. Virus particles formed in nonsuppressor cells by several classes of ø6 nonsense mutants contained the lysin activity; however, particles formed by a mutant (unable to make proteins P5 and P11) had very low activity; high activity was produced when particles were formed in a suppressor host. A study of the time course of the appearance of the lysin during infection showed that it appeared and increased in cells infected with wild-type virus and in suppressor cells infected with a mutant of class 511, but it did not increase in nonsuppressor cells infected with the class 511 mutant. It is concluded that protein P5 is a component of the lysin and that the role of its activity is in both early and late stages of infection. In particular, the lysin may be necessary for the passage of the infecting core of the virion through the cell wall of the bacterium, as well as in the final lysis necessary for the liberation of progeny phage. A mutant of the virus that produces a larger-than-normal protein P10 does not induce normal lysin activity in host Pseudomonas phaseolicola HB10Y, although it does in strain ERA Pseudomonas pseudoalcaligenes. This indicates that protein P5 is probably not sufficient for lysin activity, but the nature of the interaction between P5 and P10 is unknown.