Structure and assembly of the bacteriophage T4 head are described as revealed by results obtained in this laboratory. Subunit arrangement of the major coat protein, soc and hoc in the head shell has been determined (Figs. 2 and 21). Two new approaches for studying the assembly pathway are presented: wild type infection at 19 degrees C and gene 23 cold sensitive mutants. We propose an assembly pathway in which the prehead is formed in one direction, starting from the neck and ending at the distal cap. The processes of proximal and distal capping of the head shell are distinctly different. Novel structural intermediates such as a naked core and cup-like particles are shown. A model on head length determination is presented, based on negative control by the core on distal capping. Interesting aspects in the maturation of the head are reviewed. An overall scheme of the head assembly is shown in Fig. 20. Comparative studies among Teven and RB phages showed that the particle morphology is strictly conserved while certain so-called non-essential and essential proteins are significantly varied. A phylogenetic relation among these phages is constructed from calculation of distances (Fig. 25).