Many small peptides can be displayed on every copy of the major coat protein in recombinant filamentous bacteriophages but larger peptides can only be accommodated in hybrid virions mixed with wild-type protein subunits. A peptide insert of 12 residues capable of display at high copy number in a hybrid virion was found to be incapable of supporting recombinant virion assembly, a defect that could not be overcome by over-expressing leader peptidase in the same Escherichia coli cell. In contrast, over-expressing leader peptidase did increase the copy number of two 9-residue peptides that were poorly incorporated into hybrid virions. The factors that limit peptide display are varied and not restricted to the early stages of viral assembly.