The cDNAs coding for murine interleukin-4 and -5 have been expressed using the baculovirus AcNPV as a vector in insect cells. Interleukins are secreted into the culture medium of virus-infected insect cells at high levels. Recombinant baculoviruses were isolated using a simple and fast selection scheme based on assays for interleukin activity. The cloning strategy described should be generally applicable to the production of any interleukin or other proteins with biological activity in the baculovirus system.