In a study of mouse encephalomyelitis the following observations were made:—
1. A definite relationship exists between the amount of virus inoculated intracerebrally and the length of the incubation period.
2. The reciprocal of the incubation period was found to be approximately proportionate to the logarithm of the amount of virus inoculated. The relationship can, therefore, be given a simple mathematical form.
3. This fact provides a basis for a new method for measurement of the activity of mouse encephalomyelitis virus.