N-linked sugar chains of rat 3Y1 cells and tumorigenic cells derived by transfection with activated c-myc gene were quantitatively released as oligosaccharides from membrane preparations by hydrazinolysis. Structural analyses revealed that cells of both types contain bi-, tri- and tetra-antennary complex-type oligosaccharides as well as high-mannose-type oligosaccharides. However, the c-myc-transfected cells showed an increase in tri- and tetra-antennary oligosaccharides having the GlcNAc beta 1----4Man alpha 1----and/or the GlcNAc beta 1----6Man alpha 1----linkages with a decrease in biantennary oligosaccharides compared to control 3Y1 cells. The data suggest that c-myc gene has a potential role in the regulation of cellular protein glycosylation and that an elevated expression of c-myc gene in the cells leads to increased branch formation of outer chains in N-linked oligosaccharides concomitant with the acquisition of tumorigenicity.