Virulence may be expressed by a bacterial count in selected tissue of an animal host inoculated and autopsied in specified conditions. Time courses of splenic infection in DBA/2 mice intravenously challenged with (1) the virulent reference Brucella abortus strain 544 or with (2) the low virulent vaccinal B abortus strain 19 evidenced a one hundred times higher initial growth of the vaccine strain, followed by a steadily decrease that contrasts with the plateau phase of the virulent strain. Thus, in vivo phases of growth and decline behave independently in which immune mechanisms involved may differ.