A temperature sensitive secretory line, MadIA214, was selected from mutagenized Chinese hamster ovary cells that express two heterologous export marker proteins: a secretory form of the human placental alkaline phosphatase (SeAP), and the Kd heavy chain of mouse MHC class I. SeAP secretion in MadIA214 was extremely reduced at elevated temperature (40 degrees C), while the export of functional H-2Kd molecules to the plasma membrane was only slightly affected. This mutant constitutively transferred onto newly synthesized proteins a truncated oligosaccharide core, Man5GlcNAc2, which was monoglucosylated in the protein-bound form. Nevertheless, the final oligosaccharide-structures associated to mature SeAP and H-2Kd were similar in mutant and wild-type glycoproteins. The inaccessibility in MadIA214 endoplasmic reticulum (ER) of one or more components required for oligosaccharide chain elongation is supported by the reconstitution of a correct core structure, obtained after disruption of cellular compartments, but not after cell permeabilisation or blocking ER-to-Golgi transport. The increased association of the ER-chaperone BiP with immature SeAP correlated with the thermodependent decrease in SeAP secretion. The retention of incompletely folded polypeptides in MadIA214 parallels both a marked ER-dilation and an important glycoprotein degradation documented by the formation of soluble oligomannosides with one GlcNAc residue. Our data provide the first in vivo evidence that the initial step in N-glycosylation differentially governs glycoprotein maturation, transport and degradation.