The product of gene 31 (P31) of bacteriophage T4 is required for the formation of the phage capsid and its related structures. In the absence of active P31, product P23, the major component of the phage capsid, aggregates into “lumps” which sediment with the cell envelope. Temperature-shift experiments with ts-mutants in gene 31 demonstrate that the P23 aggregates can be dissolved by activated P31 and the dissolved P23 is normal, in that it can be used for incorporation into active phage. It is possible that P31 acts catalytically. Two different ts-mutants in gene 31 produce two different temperature-sensitive proteins. One is irreversibly inactivated if produced at the restrictive temperature; but when synthesized at the permissive temperature, it becomes heat stable and remains functional at the restrictive temperature. The other is reversibly affected by temperature, activated following shift to permissive temperature and inactivated if restrictive temperature is established.