Current procedures for inoculating lepidopteran larvae with polyhedrin-negative recombinant baculovirus, i.e. intracoelomic injection or coinfection with wild type virus, are laborious and can compromise final yields of recombinant protein. Herein is described a simple and efficient method for oral inoculation. Up to 100% infection was obtained when individual early fifth instar Trichoplusia ni larvae were fed a small piece of a formaldehyde-free insect diet to which 4.2 x 10(5) PFU of a polyhedrin-negative recombinant Autographa californica nuclear polyhedrosis virus (AcNPV) containing the gene for beta-galactosidase was applied. Infected larvae were identified by assaying hemolymph for beta-galactosidase activity. The maximum levels of beta-galactosidase detected in these hemolymph samples were identical to those obtained for larvae infected by intracoelomic injection. The dose of polyhedrin-negative recombinant virus recommended for intracoelomic injection of T. ni was efficacious for the oral route of inoculation.