Both spontaneous and NTG-induced mutants of Escherichia coli 594 insensitive to the lethal action of λ P gene were isolated and called rpl (resistant to Plethality). These mutants were of two types, showing different phenotypes. On type I rpl mutants, λcl- and λv1 v3 did not plate, while λvir, λcl-c17, λimm434, and λimm21 did; plasmid pMR45 carrying the λP gene could not complement λimm21P- phage in type I mutants. On the other hand, the type II rpl mutants support the growth of all the above phages including λcl-. Neither type of rpl mutation affects growth of the bacteria. © 1991.