Previous reports have demonstrated that hydrocephalus can be produced in animals by intracranial innoculation of mumps, influenza, and parainfluenza virus. A mutant measles virus produced by treatment with proflavine when injected intracranially in newborn hamsters also produced hydrocephalus due to aqueductal stenosis in 76%.1 Other mutant strains of measles virus did not produce hydrocephalus. The authors point out that "virus mutants should be considered potentially more hazardous than the parental strain."