Studies presented here show that heparin alters immunoglobulin expression by murine pre-B cell lines and normal Ly-1 (CD5+) B cells. Previous studies have shown that pre-B cell lines 70Z/3 and NFS-5.3 express mu heavy chains in the cytoplasm and a small amount on the cell surface. Both these cytoplasmic and surface mu are disulfide-linked to omega (lambda 5) surrogate light chains and are noncovalently associated with iota (Vpre-B) variable region-like proteins. We show that culturing 70Z/3 with heparin reduces the amount of the membrane-form mu (micron) on the cell surface. Culturing NFS-5.3 with heparin similarly decreases the membrane-form mu; however, it increases the surface level of a pentameric mu molecule containing secreted-form mu (microS) heavy chains, disulfide-linked omega (lambda 5) chains, and noncovalently associated proteins. Culturing peritoneal B cells with heparin also increases the production of the secreted-form microS, detectable in this case by the secretion of classical pentameric IgM. Similarly, injecting heparin intraperitoneally increases IgM secretion by peritoneal Ly-1 B cells. Thus heparin could influence pre-B cell and B cell differentiation and function.