Anti-CD19 does not induce the internalization of P-gp. Cells (10⁶) were incubated for 30 minutes on ice in the absence or presence of 15 μg anti-CD19, anti-CD22 (negative control), or anti–P-gp (positive control). The excess unbound antibody was removed by 2 washes in PBS, and then the cells were incubated at 37° C for 4 hours. The level of cell surface P-gp was then detected using PE-UIC2 and cells were analyzed by FACS. WT indicates wild type Namalwa cells.