The effects of amber mutations in the 24 known essential genes of phage T1 on phage-directed protein synthesis were examined by SDS-polyacrylamide gel electrophoresis of radiolabelled polypeptides from infected non-permissive cells. Mutations in nine genes (genes 1, 2, 3, 3.5, 5, 7-8, 10, 13.3 and 15) each resulted in the failure to synthesize a single polypeptide. Since the synthesis of each of these polypeptides was at least partially restored in permissive infections and wholly restored in am+ revertant infections we conclude that the affected polypeptide is the primary gene product. Mutations in genes 13.7, 16 and 17, which are required for head formation, are all defective in the synthesis of a 68000 mol. wt. non-structural polypeptide. Mutants in the head genes 13 and 14 fail to cleave P7p to P7, the major structural component of proheads. Gene 14 mutants also fail to make a 45000 mol. wt. non-structural polypeptide which appears to be involved with the gene 13 product in the P7p cleavage reaction. The protein products of the DNA genes 1, 2 and 3.5 are, as expected, synthesized predominantly early in infection whereas those of the remainder, which determine head and tail formation, are made in greater amounts late in infection. Gene 3, a tail gene which has been mapped within the 'early' DNA gene cluster, codes for a 75000 mol. wt. polypeptide synthesized predominantly late in infection. This observation suggests that the early genes 1, 2, 3.5 and 4 do not form an operon under the control of a single early promoter.