Interaction of CD4 with MHC class II molecules plays a crucial role during thymic development and activation of singie-positive CD4 T lymphocytes. The quantitation of this interaction is, therefore, important for understanding the role of CD4 during these events. To this end, we have developed a rosette assay, which enabled us to study this molecular interaction. By coupling soluble mouse CD4 onto beads, we could show specific binding of CD4 to MHC class II molecules on A20 B lymphoma cells. These binding studies revealed an extremeiy low affinity (Ka ≤ 104 M−1 between CD4 and MHC class II molecules in mouse.