The relative binding activity of RFB4(dsFv)PE38 compared with native antibody on CA46 cells. Whole antibody and recombinant immunotoxin were used to compete for binding of trace amounts of ¹²⁵I-labeled RFB4 IgG. Counts competed are expressed as a percentage of counts from cells that were incubated without any competitor. (□), RFB4 IgG; (▴), RFB4(dsFv)PE38.