This study was designed to determine the time in the intracellular life of immunoglobulin when the carbohydrate moieties are added. Plasma cells from a mouse myeloma tumor were exposed to glucosamine-(3)H (a "bridge" sugar), galactose-(3)H, or leucine-(3)H. With each of the above isotopes, the percentage of total radioactive immunoglobulin that has been secreted after different periods of labeling and the extent to which puromycin prevented incorporation into immunoglobulin were determined. The results indicate that both galactose and glucosamine (in its N-acetyl form) become covalently incorporated into immunoglobulin G late in its intracellular life and suggest that glucosamine is also added onto nascent polypeptide chains (i.e., on polyribosomes).