Intracellular nucleocapsids, virtually free of host proteins and nucleic acids, were purified from African green monkey kidney Vero cells acutely infected with several strains of mumps virus. Viral RNA obtained from the nucleocapsids was predominantly a single 50S species, with 28S RNA occurring as a secondary peak in the RNA profile. Nucleocapsids yielded .apprx. 9 .mu.g 50S RNA/109 infected cells. The 50S RNA was partially self-complementary, indicating that mumps viral nucleocapsids include full-length plus (+) sense antigenomes. Self-annealing and competition hybridization assays with 50S RNA suggest that 28S RNA includes duplexes consisting of full-length (+) and (-) sense 50S RNA. Properties of the viral RNA did not vary with the method of RNA isolation,time post-infection at which nucleocapsids were harvested nor the strain of mumps virus.