Internalization kinetics of hBU12 and hBU12-vcMMAE on NHL and ALL tumor cell lines. Internalization kinetics of hBU12 and hBU12-vcMMAE was determined by flow cytometry of CD21^low (A) Nalm-6 and (B) Ramos cells. The majority of the compounds disappeared from the cell surface within 1 to 2 hours after incubation. Internalization kinetics of hBU12 and hBU12-vcMMAE on CD21^high (C) Raji and (D) Daudi cells. Similar to our findings with CD21^low cells, the majority of the compounds internalized within 1 to 2 hours of incubation of CD21^high cell lines. However, the fraction representing the noninternalized compounds was higher in CD21^high compared with CD21^low cells after 4 hours of incubation.