The polyhedral inclusion bodies of a nuclear polyhedrosis virus from the spruce budworm Choristoneura fumiferana were purified by fluorocarbon treatment, sucrose columns, sucrose density gradients, and equilibrium centrifugation. The preparation yields a single and homogeneous band in potassium tartrate gradients at a density of 1.193 g/ml. The virions were released from the polyhedra by alkali treatment and purified by either rate zonal or equilibrium centrifugation in sucrose gradients. The purified virions, which band at a density of 1.265–1.300 g/ml in sucrose, were infectious to the spruce budworm larvae and their mean LD50 was 1.09 ± 0.1 μg of virus. However, viral nucleocapsids, released by a nonionic detergent, Nonidet P-40, had a density of 1.315 g/ml and were noninfectious by either per os inoculation or intrahemocoelic injection.