Expression of CD19 and P-gp as determined by the binding of anti-CD19 (HD37) and anti–P-gp (UIC2) MAbs to Namalwa/MDR1 cells. (A) Cell surface P-gp expression on Namalwa (dashed line) versus Namalwa/MDR1 (solid line), detected with PE-mouse anti–P-gp (UIC2) and analyzed on a FACScan, and (B) the corresponding total P-gp protein level, detected by Western blotting of the 1% Triton X-100 cell lysates (100 μg protein/lane). The binding affinities of HD37 (○) and UIC2 (•) for P-gp on Namalwa/MDR1 cells were calculated from the saturation curve expressed as (C) percent positive cells and (D) mean fluorescence intensity (MFI).