Internalization of antibody-bound wild-type CD33 in primary AML blast cells. Primary AML blast cells were labeled with unconjugated hP67.6 on ice water before the cells were incubated at 37°C in antibody-free medium to allow internalization of antibody-bound CD33 up to 4 hours as indicated. Subsequently, remaining cell surface–associated hP67.6 was detected with biotin-conjugated mouse anti–human IgG₄ monoclonal antibody and a streptavidin-PE conjugate. Shown is the percentage of internalized hP67.6 relative to cells kept at 0°C.