A bacterial mutation affecting lambda DNA replication, called groPC756, has been mapped between the thr and leu bacterial loci. Most of the parental lambda DNA does not undergo even one round of replication in this host. Lambda mutants, call pi, which map in the lambda P gene are able to overcome the inhibitory effect of the groPC756 mutation. It is shown that the mutation at the groPC locus also interferes with bacterial growth at 42 degree C. A lambda-transducing phage,carrying the groPC+ allele, was isolated as a plaque-former on groPC756 bacteria. Upon lysogenization, it restores both the gro+ and temperature resistant phenotypes.