Abstract When centrifuged in sucrose density gradients, Prunus necrotic ringspot virus separated into three zones of nucleoprotein particles with sedimentation rates in 0.03 M Tris, pH 7.5, of 72, 90, and 95 S. Bottom particles were slightly infectious; middle and top particles were noninfectious. Mixing of bottom and middle particles markedly increased infectivity, but additions of top particles decreased infectivity. A host range determinant conditioning infection of Vigna sinensis was carried in bottom particles. In 8 M urea, four bands of RNA were resolved by polyacrylamide-gel electrophoresis. After formaldehyde modification, three bands were resolved with molecular weights estimated as 0.95 × 10 6, 0.66 × 10 6, and 0.31 × 10 6.