We have cloned the phage Mu A gene, with and without the gene ner, under the control of the pL promoter of phage lambda in a multicopy plasmid vector. We demonstrate that plasmid-carrying cells are able to support growth of superinfecting Mu A am phages in a temperature-dependent fashion in a host strain carrying a defective lambda prophage which specifies the cI857-coded lambda repressor. In addition, we show that the presence of the ner gene reduces the efficiency of plating of the superinfecting phage. Analysis of proteins specified by the cloned Mu fragments indicates that two proteins, 70 and 33 kDal, are synthesized. The level of synthesis, compared to that of the vector-encoded beta-lactamase, was found to increase with temperature. This indicates that their transcription is driven by the pL promoter. The Mr of the 70-kDal protein is identical to that previously observed for pA.