1. Laboratorio de Microbiologia Experimental, Instituto Nacional de Investigaciones Pecuarias, Km. 15½ Carretera a Toluca, Palo Alto, D. F., Mexico
2. Arbovirus Unit, Virology Section, Laboratory Division, Center for Disease Control, U. S. Public Health Service, Atlanta, Georgia 30333
3. Laboratory Investigations Unit, Viral Zoonoses Section, Epidemiology Program, Center for Disease Control, U. S. Public Health Service, Lawrenceville, Georgia 30245