A virus recognized by its capacity to multiply in grivet monkey kidney cultures and to interfere in such cultures with subsequent multiplication of ECHO-11 virus, was isolated regularly from military recruits with rubella, but rarely from those hospitalized at the same time and place with scarlet fever. The newly recovered virus multiplied consistently in primary and grivet kidney cultures. The virus was filterable, sensitive to heat and organic solvents, and immunologically distinct from a number of other chloroform sensitive agents. Neutralizing antibody often was present in acute phase sera of patients with rubella and regularly increased significantly during convalescence in patients yielding the virus.