Ex vivo detection of rituximab on the surface of CLL cells. (A) Negative control and lack of staining noted when isotypic control antibodies were used to stain isolated lymphocytes from a patient with confirmed CLL. CD20 was detected in the gated lymphocytes before adding rituximab (B), and there was no detectable rituximab on the surface of cells using antimouse Ig antibodies (C). After adding rituximab, the cells became completely negative for CD20 (D) and strongly positive for rituximab using antimouse Ig sera (E). The percentage of cells in each quadrant is shown in each plot.