Down-modulation of CD20 increases with increasing incubation time with plasma. Adding rituximab to cells from a patient with CLL masks the detection of CD20 (A) and shows strong positivity for rituximab (antimouse Ig; B). Incubating the cells with rituximab and the patient's plasma for 30 minutes partially reduced the detection of CD20 (C) and rituximab is easily detected on the surface of cells (D). The same cells when analyzed 1 hour after incubation with rituximab and plasma became less positive for CD20 (E) and remained strongly positive for rituximab (F). After 120 minutes the cells became completely negative for both CD20 (G) and rituximab (H). The percentage of cells in each quadrant is shown.