Susceptibilities to attenuated poliovirus of cynomolgus monkeys were assessed by the intraspinal inoculation method with the same single bulk lot as the reference for each of the three poliovirus types. The lesion-inducing virus dose in the spinal cord of 50% of monkeys inoculated (LID50) varied widely from 10(6.4) to lower than 10(2.5), the difference being as much as 10,000-fold, depending on the birth place of the monkey. The birth places of the monkeys used were traced and categorized into the following three districts; the peninsular region, Indonesia and the Philippines. The LID50 value of the attenuated poliovirus in the monkeys from the Philippines were much lower than those in the monkeys from the other two districts.