In 38C B lymphocytes, membrane IgM is expressed on the surface, whereas secretory IgM (sIgM) is rapidly degraded. Here, we localize this degradation and characterize the proteases involved in this process. Upon treatment with brefeldin A, degradation of sIgM in 38C cells was strongly inhibited, as was secretion from the sIgM-secreting D2 hybridoma. Moreover, the brefeldin A-induced Golgi resorption resulted in galactosylation of sIgM and partial resistance to endoglycosidase H. However, sIgM avoided degradation neither due to modified terminal glycosylation nor as a consequence of the brefeldin A-induced altered milieu of the endoplasmic reticulum. When these modifications were prevented by inhibiting retrograde transport with nocodazole or by abrogating terminal glycosylation with swainsonine, sIgM was still rescued from degradation. The unaffected breakdown in the presence of nocodazole also argued against recycling of sIgM to be degraded in the endoplasmic reticulum. Furthermore, upon removal of brefeldin A, degradation of galactosylated sIgM resumed in 38C cells, as did secretion from D2 cells. These results indicate that functional export of proteins from the endoplasmic reticulum is a prerequisite for sIgM degradation. Biochemical characterization of this novel postendoplasmic reticulum/pre-trans-Golgi proteolytic pathway included application of inhibitors to a broad spectrum of proteases. Among the compounds tested, only calpain inhibitor I exerted strong inhibition. The involvement of cysteine protease(s) in the degradation of sIgM was corroborated by the inhibitory effect of diamide. We conclude that B lymphocytes avoid secretion by active and selective targeting of sIgM to a developmentally regulated postendoplasmic reticulum degradation pathway in which degradation is mediated by a cysteine protease.