The arboviruses Oriboca and Restan of the serologic Group C, and Manzanilla of the Simbu group were grown in BHK-21 cells, purified by ultracentrifugation, and demonstrated by electron microscopy. The particles of all three viruses were spherical and had an envelope. Oriboca had an average diameter of 50 mµ, Restan of 48 mµ, and Manzanilla of 51 mµ. Manzanilla was inactivated in 24 hours at 25°C, whereas Oriboca was more resistant and inactivated only after 48 hours at 40°C. All activities were measured by the plaque method.
Permanent address: Escuela de Biología, Departamento de Microbiología, Universidad del Oriente, Cumaná, Venezuela.