The DNA-protein complex DNA-gp3 of ∅29 is efficiently packaged into purified proheads with the aid of plasmid-derived gp16. The filled heads can be assembled to phage by addition of an extract providing the products for neck-tail assembly (Bjorns et al., J. Virol. 50:772, 1984). However, purified proheads lost their competence to package DNA-gp3 upon storage for 2 months at 4°C. Competence was restored by complementation with extracts of certain mutant-infected cells, and these experiments demonstrated that late proteins were not involved; restoration obtained with 4-8-14- infected cells was indistinguishable from that obtained with 7-8-14- infected cells. 2-8-14- and 3-8-14- extracts restored about one-third of the capacity to package exogenous DNA-gp3. A 1-8-14- extract restored activity to package 20.6% of the DNA-gp3 added, but phage were not produced.