There is reason to believe that during the serial transmission of a single strain of poliomyelitis virus through monkeys, periodic changes in its infectivity occur (Flexner and Lewis (1), Flexner et al. (2), Flexner and Amoss (3)). It is not surprising, therefore, that the infectivity of the different strains in common use show no agreement. This is revealed in the experiences of various workers. Amoss (4), for example, states that as little as 0.001 cc. of a 5 per cent emulsion can infect monkeys, while Fair-brother (5), infected with 0.003 cc. and Stewart and Rhoads (6) with 0.005 cc. of a 5 per cent emulsion. On the other hand, Shaughnessy, et al. (7) and Rosenow (8) failed to infect monkeys with as much as 1 cc. of a 5 per cent emulsion. Furthermore, as is shown in an accompanying paper, poliomyelitis virus is not uniformly distributed throughout the brain and cord.