Complete kit for expressing recombinant proteins at high levels. The BacPAK System uses the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) to produce target proteins in insect cells. More than 90% of the viruses produced by the transfected cells carry the target protein. The expressed recombinant protein is similar in structure, biological activity, and immunological reactivity to the naturally occurring protein because insect host cells provide post-translational processing similar to that of mammalian cells.