Human core 1 β3-galactosyltransferase (C1β3Gal-T) generates the core 1 O-glycan Galβ1-3GalNAcα1-Ser/Thr (T antigen), which is a precursor for many extended O-glycans in animal glycoproteins. We report here that C1β3Gal-T activity requires expression of a molecular chaperone designated Cosmc (core 1 β3-Gal-T-specific molecular chaperone). The human Cosmc gene is X-linked (Xq23), and its cDNA predicts a 318-aa transmembrane protein (≈36.4 kDa) with type II membrane topology. The human lymphoblastoid T cell line Jurkat, which lacks C1β3Gal-T activity and expresses the Tn antigen GalNAcα1-Ser/Thr, contains a normal gene and mRNA encoding C1β3Gal-T, but contains a mutated Cosmc with a deletion introducing a premature stop codon. Expression of Cosmc cDNA in Jurkat cells restored C1β3Gal-T activity and T antigen expression. Without Cosmc, the C1β3Gal-T is targeted to proteasomes. Expression of active C1β3Gal-T in Hi-5 insect cells requires coexpression of Cosmc. Overexpression of active C1β3Gal-T in mammalian cell lines also requires coexpression of Cosmc, indicating that endogenous Cosmc may be limiting. A small portion of C1β3Gal-T copurifies with Cosmc from cell extracts, demonstrating physical association of the proteins. These results indicate that Cosmc acts as a specific molecular chaperone in assisting the folding/stability of C1β3Gal-T. The identification of Cosmc, a uniquely specific molecular chaperone required for a glycosyltransferase expression in mammalian cells, may shed light on the molecular basis of acquired human diseases involving altered O-glycosylation, such as IgA nephropathy, Tn syndrome, Henoch-Schönlein purpura, and malignant transformation, all of which are associated with a deficiency of C1β3Gal-T activity.