If A is the average density of virus particles per unit volume and am(a)l, m= 2, 1,0, 1,2,) are the dilution levels, the proportion of eggs remaining sterile at dilution am (if eggs are used as the material to be infected) will be e~Xam if the ordinary assumptions of dilution assay are valid. In many cases this is not true because the eggs vary amongst themselves in their infectibility. If p is the probability of any one particle (when present) being infective for an individual egg, and if p does not vary from particle to particle the probability of the egg remaining sterile is e~pXam. If p varies from egg to egg so that it has a probability distribution/(p), the probability of an egg chosen at random remaining sterile is