U937 cells were incubated with the CD98 antibodies at a wide variety of concentrations (between 0.5 and 30 μgm/mL) and tested both for binding to U937 and for the ability to induce aggregation. The data shown here were selected so as to show aggregation at a concentration that gave comparable cell-surface binding, with mean fluorescence intensity (MFI) between 100 and 200. Binding to the U937 cells was measured by indirect immunofluorescence and flow cytometry. MFI was calculated by means of WIN-MDI software on a minimum of 5000 cells. Aggregation at 6 hours was measured by means of the quantitative assay described in “Materials and methods.” The results are shown as mean for triplicate cultures ± SEM.