We have identified the products of four of the six genes involved in bacteriophage T4 tail fibre assembly by sodium dodecyl sulphate-acrylamide gel electrophoresis of tail fibre mutant lysates and particles purified from them. Two large polypeptides, a 150,000 molecular weight species which is the product of gene 34 (P34), and a 120,000 molecular weight species which is the product of gene 37 (P37), are the major structural components of the fibres. Two smaller polypeptides, the products of genes 38 and 57, act in the conversion of the large structural polypeptide chains into morphological and antigenic half fibres. P38, molecular weight 26,000, does not appear to be a structural protein of the phage. In its absence, P37 is synthesized but remains unassembled. P57 plays a pleiotropic role in phage assembly: in its absence, P37 and P34 are both synthesized, but neither is assembled into fibres, and P12, a 60,000 molecular weight protein of the baseplate, is not incorporated into baseplates. The state of these unassembled polypeptide chains from 38^- and 57^- lysates can be distinguished from their state in wild-type lysates by two criteria: (a) they are soluble in sodium dodecyl sulphate at room temperature, whereas normal fibres and phages require heating for solubilization, and (b) they are concentrated in the low-speed pellet fractions of the lysates, suggesting that they are either aggregated, or bound to the cell envelope.

A gene 36 amber mutation depressed the synthesis of P37 and a gene 37 amber mutation depressed the synthesis of P38, suggesting that these three genes are cotranscribed.

These findings allow the formulation in greater detail of the early stages of the fibre assembly pathway.