Internalization of CD20 and rituximab at the early stage of therapy with rituximab. Cells from a patient with CLL were positive for CD20 (A) and negative for rituximab (B) before therapy. They became negative for both CD20 (C) and rituximab (D) 24 hours after rituximab therapy. The same cells showed cytoplasmic positivity for both CD20 (E) and rituximab (F), confirming internalization of the CD20/rituximab complexes. Isotypic controls are shown. The percentage of cells in each quadrant is shown.