1. Rhesus monkeys inoculated with canine distemper are relatively or completely immune to experimental poliomyelitis during the first 2 weeks of the distemper.
2. Monkeys convalescent from distemper are not resistant to experimental poliomyelitis.
3. Two monkeys vaccinated with distemper virus responded to poliomyelitis in a modified manner.
4. Distemper antiserum did not influence the course of experimental poliomyelitis in rhesus monkeys.
5. Equine encephalomyelitis and vaccinia encephalitis showed no sparing effect on the course of experimental poliomyelitis.
6. The concurrence of distemper and poliomyelitis in monkeys seems to represent a new immunity mechanism in the virus field.