Stimulation of bone-marrow-derived lymphocytes by locally concentrated concanavalin A involves first an increased intracellular synthesis of total protein and of immunoglobulin M at 10-14 hr, followed by the initiation of secretion of protein and of IgM at 24-30 hr after stimulation. Differentiation into immunoglobulin-producing and -secreting cells after stimulation is manifested by (a) a rate of IgM synthesis that increases continuously over total protein synthesis, and (b) an increased rate of IgM secretion with concomitant decrease of secretion of other proteins. Within stimulated bone-marrow-derived cells, IgM molecules were found mainly in their 7S form, together with some light-mu-heavy-chain precursors. Small amounts of polymerized 19S IgM are found associated with the cells. Only fully assembled 19S IgM was found secreted into the extracellular medium. Polymerization into the pentamer therefore takes place shortly before, or simultaneously with, secretion. Intracellular 7S IgM contains only glucosamine and mannose residues, with traces of fucose and galactose residues. Secreted 19S IgM has the full complement of carbohydrates with glucosamine, mannose, fucose, galactose, and N-glycolylneuraminic acid residues. During polymerization and secretion from the cells, IgM molecules therefore acquire all of the semiterminal and terminal residues of their carbohydrate groups.