Syk is essential for the engulfment of zymosan but not for its attachment to CR3. Macrophage-like differentiated Day3-HL60, DN-Syk/HL (clone 8), Syk-siRNA/HL (clone 6), control siRNA/HL (clone 4), or Flag-rescue–Syk/Syk-siRNA/HL cells were treated with serum-opsonized fluorescent zymosan for 30 minutes at 37°C and observed with a fluorescence microscope before and after trypan blue staining. The cells treated with zymosan were examined by the quenching assay described above. In each culture plate, more than 100 cells and total zymosan particles (inside and outside the cell; seen before quenching) and bright zymosan particles (inside the cell; seen after quenching) were counted and the average number of zymosan particles per cell was calculated together with SD of triplicate experiments. The counts of outside the cell were obtained as the result of subtraction (inside and outside minus inside) in each plate.