Phage-related structures found in wild-type and mutant T1 infections were examined by sedimentation analysis, electron microscopy, and polyacrylamide gel electrophoresis. Phage-particle polypeptides P7 (molecular weight 33,000) and P11 (molecular weight 16,000) were identified as major head proteins and P10 (molecular weight 26,000) was shown to be the major tail protein. A DNA-free head-like structure containing P7 but not P11 was present in wild-type and a number of mutant infections. In view of its possible role as a precursor to mature heads, this structure was termed the prohead. Mutants in two genes, am10 (gene 13) and am45 (gene 14), synthesised only tails in nonpermissive infections. Mutants in eight of the head genes, am23 (gene 4), am283 (gene 13.3), am216 (gene 13.7), ts257 (gene 14.5), am11 (gene 15), am4 (gene 16), am7 (gene 17), and am30 (gene 18), synthesised proheads and tails but not other structures. am37 (gene 12) synthesised proheads, tails, and empty heads. Studies with a tail-defective mutant suggested that empty heads were produced from the breakdown of DNA-filled heads. P11, the T1 gene 13.3 product, is analogous to the phage lambda gene D protein: both are major capsid proteins which appear to stabilise the later stages of head filling. These data are reviewed according to the general principles established for phage-head assembly and a tentative pathway for T1-head assembly is proposed.