Measles virus was propagated in monolayer cultures established from brain tissue of a patient with subacute sclerosing panencephalitis. Syncytial cells were rendered fluorescent with measles specific antiserums only, by means of an indirect technique. The ultrastructural appearance of the microtubular aggregates was identical in brain tissue and in the cultured cells. Fusion experiments produced a cytopathic effect in humnan embryonic kidney and VERO cell cultures. The virus was identified by hemiagglutination-inhibition, but only in the supernatant of disrupted cultured cells.