Increasing evidence shows that calnexin, a membrane-bound chaperone in the endoplasmic reticulum, is a lectin that binds to newly synthesized glycoproteins that have partially trimmed N-linked oligosaccharides. It specifically attaches to core glycans from which two glucoses have been removed by glucosidases I and II. Several recent reports suggest, however, that it can also bind to proteins devoid of N-linked glycans. To investigate the extent of glycan-independent binding, we have analyzed two mutant cell lines (Lec 23 and Pha(R) 2.7) that are unable to process the core glycans because they lack glucosidase I or glucosidase II, respectively. In contrast to parental cell lines, calnexin binding of substrate proteins was found to be virtually nonexistent in these cells. Neither cellular nor viral proteins associated with the chaperone. It was concluded that glycans are crucial for calnexin association and that the vast majority of substrate proteins are therefore glycoproteins.