The late stages of bacteriophage T 4 maturation apparently involve the interaction of preformed components, since viable phage can be formed in vitro from viral parts such as heads, tails and tail fibres which accumulate in bacteria infected with mutant phage (Edgar & Wood, 1966). Similar conclusions have been reached for phages λ (Weigle, 1966) and P 22 (Israel, Anderson & Levine, 1967). The experiments described here show that with wild-type phage T 5 viral heads are precursors of mature phage in vivo. Phage T 5 and 32P-labelled phage were prepared by growth on Escherichia coli b/2 as described by Smith & Burton (1966). Carrier-free 32P-phosphate and [6-3H]thymidine, specific activity 17.7 c/m-mole, were obtained from the Radiochemical Centre, Amersham. In experiments involving artificial lysis, the host was E. coli c 3000 since this behaved more reproducibly than strain b/2 in trial experiments.