Observations are reported on the pathogenicity of the Col. SK-MM-EMC group of viruses for cynomolgus monkeys. Col. SK, MM, High Lansing and EMC virus, harvested from remote rodent passages, produce typical poliomyelitis in intracerebrally infected cynomolgus monkeys. Two strains of highly virulent Theiler virus, GDVII and FA, are non-pathogenic for cynomolgus monkeys. The Col. SK-MM group of virus can be carried in cynomolgus monkeys only for a limited number of passages. Judging from serol. and biol. characteristics, monkey passage fails to bring about a permanent conversion of Col. SK virus into Y-SK virus.