Activity of syncytiogenic measles virus was found in culture of brain cells from a patient who died from subacute sclerosing panencephalitis (SSPE) of 10 years' duration. Cocultivation with cells susceptible to measles virus and repeated passage of the cocultures failed to release free measles virus from the human brain cells. Suspensions of the SSPE brain-cell cultures were encephalitogenic in ferrets, and the syncytiogenic measles-virus activity was recovered in brain-cell cultures from sick ferrets. The multinucleated syncytia in ferret cultures looked identical to those in the human cultures and contained measles-virus antigen both in the cytoplasm and in intranuclear inclusion bodies. The encephalitogenic activity could be passed serially in ferrets and correlated with a syncytiogenic measles-virus activity in the cultures of ferret brain. The encephalitogenic activity was retained in freeze-thawed suspensions of ferret-brain cultures that apparently contained no viable cells and no measles-virus activity in tissue culture.