Degradation of IgM μ heavy chains in light chain-negative pre-B cells is independent of vesicular transport, as is evident by its insensitivity to brefeldin A or cell permeabilization. Conversely, by the same criteria, degradation of the secretory μs heavy chain in light chain-expressing B cells depends on vesicular transport. To investigate whether the presence of conventional light chains or the developmental stage of the B-lymphocytes dictates the degradative route taken by μ, we express in 70Z/3 pre-B cells either λ ectopically or κ by lipopolysaccharides-stimulated differentiation into B cells and show their assembly with μ heavy chains. The resulting sensitivity of μ degradation to brefeldin A and cell permeabilization demonstrates that conventional light chains, a hallmark of B cell differentiation, are necessary and sufficient to divert μ from a vesicular transport-independent to a vesicular transport-dependent degradative route. Although both routes converge at the ubiquitin-proteasome degradation pathway, only in light chain-expressing cells is vesicular transport a prerequisite for μ ubiquitination.