Six of seven hfl mutations of Escherichia coli K12, characterized by high frequencies of lysogenization by phage lambda and lambdacIII mutants, are shown to be tightly linked to, but not within, the purA locus. All six hfl mutations are recessive to wild type in hfl(+)/hfl merodiploids and all lie in a single complementation group, located just counterclockwise from the purA locus. All six mutations confer a slightly increased resistance to penicillin and rifamycin and a slightly increased sensitivity to sodium dodecyl sulfate. Some cases of intragenic complementation and intragenic recombination were observed. It is argued that the hfl(+) gene determines the synthesis of a protein which antagonizes lysogenization by phage lambda. It is further argued that the function of the lambdacIII gene product is to negate the antagonistic effect of this hfl(+) protein.