A comparative study was made on the glycoform of O-glycan from human myeloma immunoglobulin A1. By gas-phase hydrazinolysis, O-glycan was released from its hinge portion. The released oligosaccharide was pyridylaminated and separated by a two-dimensional analytical method of gel filtration and reverse-phase HPLC. Four major pyridylamino derivatives (P1-P4) were obtained. The neutral component (P4) among them was identified as Gal beta 1,3GalNAc-PA by cochromatography with an authentic standard pyridylamino sugar. The desialylation of the other components indicated the largest P1 and middle size P2 components possibly corresponded to a disialylated structure, NeuAc alpha 2,3Gal beta 1,3(NeuAc alpha 2,6)GalNAc-PA, and a monosialylated component, NeuAc alpha 2,3Gal beta 1,3GalNAc-PA, respectively. The structural assignment of P3 is still incomplete. Four similar components were also detected in bovine fetuin whose relative content (P1:P2: P3:P:4) was 16:43:19:22. The relative content (%) of P1-P4 (glycoform) in IgA1 from the healthy control was 10.1 +/- 3.3, 48.2 +/- 4.6, 7.0 +/- 2.6, and 34.7 +/- 4.5. The glycoform of O-glycan on IgA1 thus appears the same for any individual. Analysis of IgA1 myeloma protein indicated glycoforms distinct from those of the healthy controls. The relative content of these component could be classed as 2:8:0:90 (Type I, only one case designated as Kita), 5:24:3:68 (Type II, seven cases), and 9:41:5:45 (Type III, four cases). Thus, the results for IgA1 myeloma protein indicate that at least three glycoforms of O-glycan are possible for the IgA1 hinge structure. However, only one glycoform was found in the healthy controls.