A DNA fragment encoding the transposon Tn9 chloramphenicol acetyltransferase gene (cat) was inserted into M13 phage and pUC plasmid cloning vehicles. When the cat gene was inserted in the same orientation as the lacZ gene, two new polypeptides were produced. One polypeptide possessed chloramphenicol acetyltransferase activity, while the other expressed beta-galactosidase alpha-donor activity. Both new polypeptides were translated from a hybrid messenger RNA initiating from the lac promoter. These observations may help explain why not all inserts produce white plaques.