The proteins synthesized during the replication of phage TI in u.v.-irradiated Escherichia coli strain B have been examined by polyacrylamide gel electrophoresis of polypeptides pulse-labelled with 14C-amino acids. Up to 50 discrete bands were identified of which about 30 were sufficiently distinct to be classified in terms of time of synthesis. Three polypeptides were synthesized only during the first 6 to 8 min post infection (Class I, Early); 16 or 17 were synthesized predominantly during the later stages of replication starting from 6 to 8 min after infection (Class III, Late); three classes of proteins were made continuously, two at constant rate (Class II, Continuous), five at decreasing rate (Class IV, Early-continuous) and five at increasing rate (Class V, Late-continuous). Of the 14 polypeptides identified as structural components of the virion, three (P7, PI0 and PI) account for about 85% of the particle weight with P7 comprising 50% of the particle. P7 and PI0 appear to result from the cleavage of larger polypeptides. Preliminary studies with amber mutants suggest that normal levels of TI DNA synthesis are not required for the manufacture of late proteins and that a phage-controlled function may control the switch-off of proteins made early and the switch-on of proteins made late.