Asialo-oligosaccharides obtained by sialidase digestion of asparagine-linked acidic sugar chains of promyelocytic leukemic cells (HL-60) differed in size distribution from those of cells induced to undergo myeloid or monocytoid differentiation. High-molecular-weight oligosaccharides which are predominant in HL-60 cells, decreased slightly during myeloid differentiation and markedly during monocytoid differentiation with concomitant increase of biantennary oligosaccharides. Structural analyses revealed that the induced monocytoid cells contain a series of complex-type oligosaccharides with bi-, tri-, and tetraantennary structures and high-molecular-weight oligosaccharides with N-acetyllactosamine repeating units, which show diversity in the presence or absence of the fucose residue linked to the reducing terminal N-acetylglucosamine, of the bisecting N-acetylglucosamine residue, and of the X-antigenic determinant in their outer chain moieties. Methylation analysis of each class of oligosaccharides of HL-60 cells and of their differentiated counterparts revealed the presence of similar heterogeneity in their structures, indicating that the difference is only quantitative. The results show that the shifts in size of the oligosaccharides found in the differentiated cells are derived from changes in outer chain formation and elongation of N-acetyllactosamine repeating units, and thus suggests that the decreased expression of N-acetylglucosaminyltransferases may be involved especially in the monocytoid differentiation program.