A neutralizing monoclonal antibody of the budded phenotype of Autographa californica nuclear polyhedrons virus did not react with the occluded form of the virus as determined by neutralization, ELISA, and indirect immunoperoxidase staining. The antibody did react with the surface of infected cells in the prepolyhedra stage of cytopathic effect, the period of active virus budding. Immunoelectron microscopy indicated the antigen(s) reactive with the neutralizing antibody was present all around the viral envelope, but was more highly concentrated at the end with peplomers. Four proteins were immunoprecipitated from solubilized, radiolabeled budded virus preparations with the monoclonal antibody. The major protein had a molecular weight of approximately 64,000, while the other three were approximately 127,000, 59,000, and 49,000. All four proteins could be labeled with N-acetyl-d-[1-3H]glucosamine. This glycosylation reaction could be inhibited by tunicamycin.