Sialylated biantennary glycopeptides from human fibrinogen were analyzed with fast atom bombardment mass spectrometry. The mass spectrometric conditions used in the positive mode showed predominantly molecular ions with no fragment ions due to the loss of sialic acid. Standard mixtures of glycopeptides with zero, one, and two sialic acid residues revealed a linear relationship between ion abundance and molar fraction. The desorption efficiency varied according to the number of sialic acid residues in these biantennary glycopeptides. The relative abundance of different molecular ion species differing only in amino acid content was in agreement with chemical analysis. Sensitivity, precision, and requirements for sample preparation were assessed. Both assignment of molecular weights and quantification of individual glycopeptide species from human fibrinogen were obtained. The glycopeptides from human fibrinogen were found to consist of a mixture of equal amounts of monosialylated and disialylated species with no asialoglycopeptides.