<jats:title>ABSTRACT</jats:title> <jats:p> <jats:named-content content-type="genus-species">Streptococcus intermedius</jats:named-content> is an opportunistic bacterial pathogen secreting a human-specific cytolysin called intermedilysin (ILY) as a major pathogenic factor. This bacterium can degrade glycans into monosaccharides using two glycosidases, multisubstrate glycosidase A (MsgA) and neuraminidase (NanA). Here, we detected a stronger hemolytic activity mediated by ILY when <jats:named-content content-type="genus-species">S. intermedius</jats:named-content> PC574 was cultured in fetal bovine serum (FBS) than when it was grown in the standard culture medium. FBS-cultured cells also showed higher MsgA and NanA activity, although overproduction of ILY in FBS was undetectable in mutants <jats:italic>nanA</jats:italic> -null and <jats:italic>msgA</jats:italic> -null. Addition of purified MsgA and NanA to the FBS resulted in a release of 2.8 mM galactose and 4.3 mM <jats:italic>N</jats:italic> -acetylneuraminic acid; these sugar concentrations were sufficient to upregulate the expression of ILY, MsgA, and NanA. Conversely, when strain PC574 was cultured in human plasma, no similar increase in hemolytic activity was observed. Moreover, addition of human plasma to the culture in FBS appeared to inhibit the stimulatory effect of FBS on ILY, MsgA, and NanA, although there were individual differences among the plasma samples. We confirmed that human plasma contains immunoglobulins that can neutralize ILY, MsgA, and NanA activities. In addition, human plasma had a neutralizing effect on cytotoxicity of <jats:named-content content-type="genus-species">S. intermedius</jats:named-content> toward HepG2 cells in FBS, and a higher concentration of human plasma was necessary to reduce the cytotoxicity of an ILY-high-producing strain than an ILY-low-producing strain. Overall, our data show that blood contains factors that stimulate and inhibit ILY expression and activity, which may affect pathogenicity of <jats:named-content content-type="genus-species">S. intermedius</jats:named-content> . </jats:p>