Mutations in the groE gene of Escherichia coli, which block the correct assembly of the phage lambda head, have been previously described. Many groE mutations exert pleiotropic effects, such as inability to propagate phages T4 and T5 and inability to form colonies at 43 degrees. With the help of the EcoRI and HindIII restrictionenzymes and the appropriate phage vectors, we have constructed two lambda transducing phages, called W3 and H18, that carry the groE+ bacterial gene. Upon lysogenization by phage H18 the groE bacterial mutants recover their gro+ phenotype for both phage growth and the ability to form colonies at 43 degrees. We have identified the groE+ bacterial gene product as a protein of 65,000 molecular weight. Mutants of the W3 transducing phage that were selected on the basis of their ability to propagate on some groE mutant hosts induce the synthesis of a groE protein with altered electrophoretic mobility.