AbstractCharacteristics of two components of Oriboca virus were studied after they were separated and semipurified by velocity centrifugation. A component of low infectivity and broad cross‐reactivity in the complement‐fixation (CF) test had a sedimentation coefficient of 6–7S. Infectious Oriboca virions with hemagglutinating (HA) activity had a sedimentation coefficient of 457S. These virions cross‐reacted broadly with Murutucu viral antibody. The crossreactive, virion‐associated component and a type‐specific CF fragment were released from virions by disruption with Nonidet P‐40. These CF fragments separated by sucrose gradient equilibrium centrifugation had densities of 1.245 gm/ml and 1.181 gm/ml, respectively, and were both interpreted to be envelope structures. The type‐specific CF antigen appeared to be related to the viral hemagglutinin in that it blocked hemagglutination‐in‐hibiting (HI) antibody, whereas the cross‐reactiv