Structural components of bacteriophages T 2, T 4 and T 6 have been isolated and purified. The components have been characterized by their physical, chemical and serological properties. Each phage consists of a hexagonal head, which encloses the DNA, and a tail. The tails are constructed of three components: tail fibres, sheaths and cores. The sheaths are capable of „contraction“ and contracted sheaths show a helical structure in the electron microscope. The cores are hollow; the hole down the middle is about 30 Å in diameter. A new technique of preparation for electron microscopy, which will be briefly described, permits a study of the fine structure of intact and disintegrated phage and also a study of the assembly of the particle, by interfering with assembly by means of proflavin and mutational changes. Previous work was discussed in the light of the new findings.