Escherichia coli dnaJ- and dnaK-gene products have been identified in a system of minicells infected with the appropriate transducing lambda phages. The molecular weights of these polypeptides in dodecyl sulphate/acrylamide electrophoresis amounted to 39,000 and 77,000, respectively. Equilibrium sedimentation of minicell lysates in metrizamide density gradients has revealed that both these host proteins, indispensable for lambda DNA replication, are membrane-bound.