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Four new mutants are described whose phenotypic expression affects the length of the head of bacteriophage T4D. All mutants produce some phenotypically normal phage particles. Mutant pt2l-34 also produces at least two size classes of phage particle which have heads that are shorter than normal. The other three mutants, ptgl9-2, ptgl9-80, and ptgl91, produce, in addition to phages with normal and with shorter-than-normal heads, giant phages with heads from 1.5 to at least 10 times the normal length. All mutations are clustered near gene 23. Giant phage particles have the following properties: they are infectious and contain and inject multiple genomes as a single continuous bihelical DNA molecule of greater-than-unit length. Their frequency, relative to the total plaque-former population, increases late in the infectious cycle. They have a normal diameter, variable length, and a buoyant density range in CsCl from equal to slightly greater than that of normal phage. The arrangement of capsomers is visible in the capsids, which are composed of cleaved gene 23 protein.

Direct studies of bacteriophage reproduction have been handicapped by the fact that the cell wall of the infected bacterium presents a closed door to the investigator in the period between infection and lysis. As a result it was impossible to demonstrate the presence of intracellular phage particles during this so called latent period, and, much less, to estimate their number or to describe them genetically. This barrier has now been penetrated. It is the purpose of the first two papers of this series to describe two methods for disrupting infected bacteria in such a way that the intracellular phage particles can be counted and their genetic constitution analyzed.The first method used to liberate intracellular bacteriophage depends on the induction of premature lysis in infected bacteria by "lysis from without" which occurs when a large excess of phage particles is adsorbed on bacteria (1). It was found by nephelometric tests that T6 lysates are efficient in disrupting cells when moderately high multiplicities are used (2). The further observation was made that the addition of a large number of T6 particles to bacteria previously infected with T4, would, under some conditions, cause liberation of T4 particles before the expiration of the normal latent period of these cells. It therefore seemed hopeful that a method of reproducibly disrupting infected bacteria could be developed on the basis of this preliminary knowledge.

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