Binding of ¹²⁵I-labeled anti-CD19-Fab-SEAm fusion protein to Daudi lymphoma cells. Daudi cells (3 × 10⁵/mL) were incubated (1 hour at 22°C) with serially diluted ¹²⁵I-anti-CD19-Fab-SEAm (0.7 to 300 nmol/L). Cell-bound radioactivity is shown as a Scatchard plot and as a saturation curve (insert) after correction for background binding (B/Fnonspecific = 0.02) in each point. Scatchard analysis showed an apparent K_d of 9.0 nmol/L and approximately 1.7 × 10⁵ sites per cell (R = −.97). Each value is the mean of triplicate samples.