Plasma cells that develop in specific tumors are consistently either clonal or nonclonal. Plasma cell clonality can be assessed from the ratio of κ to λ light chain expression (top 4 panels; original magnification ×10 [J] and ×4 [W]). IHC was titered to detect the abundant cytoplasmic expression of light chains that is characteristic of plasma cells and their immediate precursors (plasmablasts) while not detecting the surface expression found on mature B cells. The bulk of plasma cells in specimen J were polyclonal, whereas the plasma cells in specimen W uniformly expressed κ light chain but did not express λ light chain and were therefore clonal. At least 3 xenografts of each specimen were stained, and all showed identical results. Typical plasma cell morphology is seen at high power in both specimens (original magnification ×100; H&E stain).