A nuclear polyhedrosis virus (NPV) isolated from Malacosoma disstria larvae was characterized biochemically and immunologically. Restriction enzyme patterns were compared with those of NPV from M. alpicola (MaNPV) and Autographa californica (AcMNPV). Polyclonal antibodies were made to M. disstria NPV and used for immunoblots. This virus isolate, designated MdMNPV, contains virions with multiple nucleocapsids, an estimated genome of 130.4 +/- 3.3 kb, and a restriction pattern distinct from that of MaNPV. Immunoblotting detected cross-reactivity among numerous structural proteins of MdMNPV and AcMNPV.