Spheroplasts of Pseudomonas BAL-31/PM2, obtained by treatment of the bacteria with lysozyme, can be infected with purified DNA from bacteriophage PM2. After 4 h of incubation the yield of progeny phage reaches a value of 107-6×107 plaque forming units/μg PM2 DNA. The yield increases linearly with the concentration of DNA over at least 3 orders of magnitude. The biological activity of double-stranded circular PM2 DNA containing one or more single-strand breaks per molecule (component II), does not differ significantly from that of intact PM2 DNA (component I). Single-stranded PM2 DNA obtained by denaturation of component II, and the irreversible alkali-denatured form of component I are also infective. © 1971 Springer-Verlag.