Abstract An interesting feature of the bacteriophage P2–P4 system is the switch in size between a large P2 (60 nm) and a small P4 (45 nm) capsid. We have investigated whether the protein processing reactions cleaving the primary translation product gpN to several capsid proteins (h1, h2, and N ∗) are involved in this switch. Using antibodies specific against gpN and its derivatives we have identified all the structural components of mature P2 and P4 particles that are derived from gpN. Our estimate of the relative amounts of gpN derivatives suggests that the previously identified minor capsid proteins h1 and h2 can only be essential structural components of the P4, and not the P2, capsid. Nevertheless, the relative amounts are similar in vivo during a P2 and a P4 infection. This indicates that the switch in head size is not caused by the presence of elevated amounts of hi and h2 during P4 morphogenesis. We have also identified the sites where gpN is cleaved to its derivatives h1, h2, and N ∗, ascertaining that the cleavage sites are the same in P2 and P4. Our results indicate that the processing reactions are not directly involved in the head size determination mechanism.