The mechanisms involved in the assembly of the head of coliphage T4 is being studied. We have found that one of the phage proteins, the product of gene 22 serves as a scaffold for the formation of an early precursor of the phage capsid. This protein is cleaved at the time the DNA is incorporated into the phage head. This cleavage has been studied in vitro using purified protein 22. We have found that it is dependent on the presence of a wild type gene 21 product in the lysate taken as a source of proteolytic activity. This proteolytic activity is associated with aberrant capsid structures found in temperature sensitive mutants in gene 23. We have also been determining the physical and chemical properties of purified protein 22. It is an acidic protein of molecular weight about 30,000. In defective lysates it is isolated as a complex with the phage internal proteins. It has a sedimentation constant of 5, a diffusion constant of 2, and contains significant amounts of alpha helix.