We studied the virulence of 20 Erysipelothrix rhusiopathiae strains in albino mice and investigated the interdependence between virulence, growth rate, and neuraminidase production. The minimal number of germs, lethal to mice (LD100) varied among the strains between 10(1) and mor ethan 10(7) per animal. The course of the disease was also very different. Some strains killed the mice rapidly within 3 days. In other strains, however, even a very high number of bacteria caused death only 7 days after infection or later, in which cases the disease became manifest long in advance by ruffled fur and glutinous eyes. A significant correlation was found between the average neuraminidase production in different media and the virulence (p equal to 99%; Fig. 2). We found a less marked dependence between growth rate and virulence (Fig. 3). Neuraminidase activity and growth rate seem to influence virulence rather independently. This is concluded from the fact, that some virulent strains are characterized by quick growth and by producing either no neuraminidase or in small quantities only. On the other hand, neuraminidase production cannot be separated completely from growth rate. An induction of neuraminidas activity in vivo was not observed, even when the bacteria persisted in the mice over a long period of time. We therefore conclude that a short term enzyme induction is not probable.