Routine analysis of suspensions of foot-and-mouth disease virus and eluates of vaccine by the isopycnic method in analytical ultracentrifuge demonstrates the important heterogeneity of the viral population. This heterogeneity increases during inactivation of the virus by formol. In view of this dispersion of the physical characteristics of viral particles it may be asked (a) whether the immunogenic value is linked to the total quantity of particles which, we know, are very different from each other and of which we can determine only the partial or total parameters or (b) whether, on the contrary, the immunogenic value is linked to the activity of marginal particles, very small in number, and perhaps even quantitatively indistinguishable by the physical methods available. However, the hypothesis of particles possessing different properties but converging in the final result cannot, a priori, be excluded. Consequently it cannot be hoped that a simple relation between the concentration in weight of a viral suspension and its immunogenic property may be determined.