Bacterial type I restriction-modification (R-M) systems present a major barrier to foreign DNA entering the bacterial cell. Previous work has shown that coliphage P1 encodes an anti-restriction system comprised of DarA and DarB (for defense against restriction), virion-associated proteins that protect DNA against type I restriction. P1darA- has reduced efficiency of plating (EOP) phenotype on E. coli strains encoding the EcoA, EcoB or EcoK R-M systems, while P1darB- has a reduced EOP on EcoB or EcoK-encoding E. coli. To further study the P1 antirestriction system, isogenic P1 deletion mutants were constructed to reconstitute the restriction phenotypes associated with darA and darB. While P1(∆darA) and P1(∆darB) produced the expected plating phenotypes, deletions of adjacent genes hdf and ddrA also gave a darA-like phenotype and deletion of ulx produced a darB-like phenotype. Interestingly, deletion of ddrB caused the phage EOP to become approximately 10-fold higher on EcoB and EcoK strains, suggesting DdrB acts as a modulator of DarB. All mutant phenotypes could be complemented in trans. SDS-PAGE and mass spectrometry of CsCl-purified virions revealed that darA, hdf and ddrA mutants fail to incorporate DarB into virions and hdf mutants fail to incorporate DarA or DarB; ulx mutants incorporate DarB inefficiently. Interestingly, virions of P1(∆ddrA), with restriction sensitivity to EcoA, still contain DarA, indicating that DarA is necessary but not sufficient for protection of phage DNA against EcoA. Analysis of phage particles showed all mutants, particularly in darA and hdf, produced high proportions of virions with heads of altered dimensions, suggesting that the incorporation of antirestriction proteins is linked to head morphogenesis. The P1 antirestriction system is more complex than previously realized and is comprised of a pathway involving multiple proteins including DdrA, DdrB, Hdf, and Ulx in addition to the previously described DarA and DarB.