In our previous study, gas-phase hydrazinolysis was used to analyze the glycoform of the 0-linked oligosaccharide of human serum IgAl. All O-linked oligosaccharide chains are known to be present in the hinge portion. However, the number of O-linked oligosaccharide chains on IgAl remained unclear. In order to determine the number of linked sugar chains, we applied matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) to the hinge glycopeptide prepared from human serum IgAl. MALDI-TOFMS did not show clear peaks, probably due to the microheterogeneity of the structure of each sugar chain. However, elimination of peripheral sialic acid and galactose residues by sequential treatment with neuraminidase and ƒÀ-galactosidase gave clear mass spectra with several sharp peaks. On the basis of these spectra, we conclude that IgAl prepared from normal human serum carries different numbers of sugar chains. There are two major populations, one contains five Ga1NAc residues and the other four Ga1NAc residues. On the other hand, the hinge glycopeptide prepared from myeloma IgAl was composed mainly of one population containing four Ga1NAc residues. Earlier, we reported incomplete glycosyl ation of IgAl isolated from the serum of an IgAl myeloma patient. In this experiment, the presence of four O-linked oligosaccharides per heavy chain of IgAl from a myeloma patient was found. The reason why only four out of five sites on the hinge glycopeptide were fully glycosylated in the IgAl from the IgAl myeloma patient is not clear.