The monoglucosylated oligomannose N-linked oligosaccharide (Glc(1)Man(9)GlcNAc(2)) is a retention signal for the calnexin-calreticulin quality control pathway in the endoplasmic reticulum. We report here the presence of such monoglucosylated N-glycans on the human complement serum glycoprotein C3. This finding represents the first report of monoglucosylated glycans on a human serum glycoprotein from non-diseased individuals. The presence of the glucose moiety in 5% of the human C3 glycoprotein suggests that this glycosylation site is sequestered within the protein and is consistent with previous studies identifying a cryptic conglutinin binding site on C3 that becomes exposed upon its conversion to iC3b.