Host controlled restriction in Escherichia coli can be relieved by pre-infecting restricting cells with modified lambda helper phages. This process, in which intact unmodified phage genomes are allowed to escape restriction attack, is mediated by a newly identified lambda function called ral. The ral gene has been located by deletion mapping between cIII and N. Efficient expression of the ral gene requires the product of the regulator gene N. Polyacrylamide gel analysis of the lambda proteins specified by the cIII-N region failed to reveal the product of the ral gene, but demonstrated that protein Ea10 is encoded by a gene located immediately to the left of ral. From these results the map order cIII-Ea10-ral-TL1-N was deduced. Ral specifically alleviates restriction in E. coli K and E. coli B, but does not affect restriction systems EcoRI, EcoRII and EcoP1. In addition, ral enhances the modification activity of the EcoK and EcoB restriction enzymes: we observed that efficient modification of progeny phages obtained by propagating unmodified lambda phages in r-m+ hosts, is dependent upon the presence of ral. We thus conclude that the ral gene product acts by modulating the restriction and modification activities of the type I restriction systems in E. coli, and the possible mechanisms will be discussed.