A total of 22 phi29-induced proteins have been resolved by slab gel electrophoresis; two of these proteins are the precursor and product fragment, respectively, in the synthesis of the neck appendage protein of the phage. The protein products of 10 out of the 17 cistrons detected in the genome of phage phi29 have been identified. Mutants in two other cistrons fail to synthesize two proteins. Mutants in six genes do not synthesize phage DNA. A cistron, probably involved in the final lysis of the infected bacteria, has been found. Mutants in this gene give place, under restrictive conditions, to delayed lysis and produce, after artificial lysis, a burst size similar or higher than that obtained after wild-type phage infection.