Calnexin and calreticulin are molecular chaperones, which are involved in the protein folding, assembly, and retention/retrieval. We know that calreticulin-deficiency is lethal in utero, but do not understand the contribution of chaperone function to this phenotype. Here we studied protein folding and chaperone function of calnexin in the absence of calreticulin. We show that protein folding is accelerated and quality control is compromised in calreticulin-deficient cells. Calnexin-substrate association is severely reduced, leading to accumulation of unfolded proteins and a triggering of the unfolded protein response (UPR). PERK and Ire1alpha and eIF2alpha are also activated in calreticulin-deficient cells. We show that the absence of calreticulin can have devastating effects on the function of the others, compromising overall quality control of the secretory pathway and activating UPR-dependent pathways.