Neuraminidase may play a role as a pathogenic factor in Erysipelothrix rhusiopathiae infection. The protective effect of active immunization with purified neuraminidase was therefore tested in an infection experiment in white mice. Mice were immunized 2, 4, 6, 8 or 10 times i.p. with Erysipelothrix neuraminidase. A control group received 10 injections with physiological saline. The infective dose varied between 7 and 7 x 10(7) cells. While all control animals infected with 7 x 10(1) germs died, the lethal effect could be reduced to 50% and 25% in animals immunized twice and 4 times, respectively. Only animals immunized 8 and 10 times were still partly protected against germ numbers of 10(3)-10(4). Germ numbers of 10(5) and more were almost always fatal even in highly immunized animals. Even a high immunization with neuraminidase could only lower the lethal by a maximum factor of 10(4) germs used.