Five mutants of Escherichia coli K12 (lam 24, lam 25, lam 26, lam 27 and lam 646) that block head formation of lambda are described. In vitro complementation tests and electron microscopy demonstrated that in these bacteria phage tails were produced normally, whereas head formation was abnormal, aberrant head-related structures being produced. In lysates prepared from lam 24, lam 25 and lam 26, monsters and empty heads without tail were the predominant structures, whereas in lysates from lam 27 and lam 646, petit lambda and empty heads were the most common structures. The five lam mutations were located in two regions on the bacterial chromosome; lam 24, lam 25 and lam 26 were near the dnaB gene and lam 27 and lam 27 and lam 646 near the lac gene. It was suggested that the former three mutants are new isolates that belong to GroE mutants, whereas the latter two comprise a new group of mutants. Analyses of phage mutants (ov mutants) that overcome the interference by the lam 646 mutation revealed that this mutation blocks normal expression of the gene E of lambda.