SUMMARYAn equation was developed to describe the relation between the numbers of infections given by inoculation of host plants with virus suspensions and the relative concentration of the suspensions. It was of the formy=N(1‐e‐m), whereywas the number of infections,Nthe maximum possible number of infections,ethe base of natural logarithms, andmthe ratio of virus units entering and causing infection to the total number of entry points, leaves, or plants (according to the unit used for judging infection).The functionmwas expanded for the particular case of dilution experiments.Methods of fitting the data obtained from dilution experiments were discussed.The data from seven dilution experiments with carefully purified samples of four viruses (infections reckoned as lesions) were fitted with equations of the form given above, and were found to agree fairly well with the calculated values.Two types of distortion in the dilution series were illustrated.The significance of the maximumNwas discussed.The same data withyas the number of leaves infected were found also to agree with the equat