The enzyme neuraminidase (EC 3.2.1.18) was found to be strongly active in different types of Streptococcus sanguis and S. viridans, and, in addition, the occurrence of the enzyme acylneuraminate pyruvate lyase (EC 4.1.3.3) was described in S. viridans. The enzyme-active bacteria strains were isolated from blood cultures of patients with septicemia. Whereas S. sanguis lost its strong neuraminidase activity after some weeks, S. viridans retained its enzyme activity for a long time in culture. Immunoelectrophoretic studies of the blood cultures of patients with streptococcal infections showed the loss of neuraminic acid in most glycoproteins of the serum, proving the in vivo action of neuraminidase. The pathogenic role of neuraminidase is discussed in streptococcal septicemia from the viewpoint of present knowledge.