Competition FACS flow cytometry. FITC-labeled 5F11 (0.25 μg/mL) was incubated with L540 cells in the presence of 10- to 20-fold excess of unlabeled antibody for 1 hour on ice. The excess probe was washed from the cells with PBS and the cell-associated fluorescence was determined by analysis using a FACScalibur instrument. All measurements were done in duplicate and error bars represent standard deviations.