9-Aminoacridine (9AA) reversibly inhibits bacteriophage T4 maturation. In the presence of 9AA, cells infected with bacteriophage T4 accumulate an intermediate similar to prohead III on the maturation pathway described by Laenmmli and Favre (J. Mol. Biol., 80, 575–599, 1973). This particle sediments at 550 S if cells are lysed under conditions that help to preserve fragile intermediates. This particle contains about half the normal complement of DBA and is bound to the replicative DNA. The completion of the DNA packaging process occurs following removal of 9AA. This was demonstrated by estimating the amount of newly synthesized DNA packaged into these particles after 9AA removal, using 5-bromo-2′-deoxyuridine as a label. Light inactivation studies showed that 75% of the particles packaged an average of at least 12% of their DNA after 9AA removal. Cleavage of the various head proteins that normally occur during maturation is not affected by 9AA, and the particles which accumulate in cells blocked with 9AA contain the same proteins found in mature heads.