Detection of CD20 by immunohistochemical stains using L26 MoAb. L26 is positive (red) in a bone marrow lymphoid aggregate from a patient with CLL treated with rituximab (A). The B cells are surface CD20^- (B), but rituximab positive (C), indicating masking of the surface CD20. In contrast, bone marrow from a different CLL patient is negative for L26 (D), surface CD20 (E), and rituximab (F), indicating absence of CD20 rather than masking. Original magnification, × 500 (A, D).