Binding of 5F11 TO CD30-expressing L540 cells. (A) Protein A-purified 5F11 or an isotype control were incubated at varying concentrations with the Hodgkin tumor cell line L540 at 4°C and detected by incubating the cells with an FITC-labeled goat anti-human IgG Fc-specific probe. 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. (B) FITC-labeled 5F11 or an FITC-labeled control human IgG (shaded) were incubated with L540 cells for 1 hour on ice in the presence (black line) or absence (gray line) of 33% human plasma. Cells were washed and analyzed by FACSCalibur.