The anti-common gamma chain (gammac) mAb CP.B8 is shown to inhibit interleukin 4 (IL-4)-dependent proliferation of phytohemagglutinin (PHA) activated T cells noncompetitively with respect to cytokine by blocking the IL-4-induced heterodimerization of IL-4Ralpha and gammac receptor chains. Affinities for the binding of IL-4 to Cos-7 cells transfected with huIL-4Ralpha, and to PHA blasts expressing both IL-4Ralpha and gammac, were used to estimate the affinity of the key interaction between gammac and the binary IL-4Ralpha.IL-4 complex on the cell surface. This affinity was defined in terms of the dimensionless ratio [IL-4Ralpha.IL-4.gammac]/[IL-4Ralpha.IL-4], which we designate KR. The results show that on PHA blasts this interaction is relatively weak; KR approximately 9, implying that approximately 10% of the limiting IL-4Ralpha chain remains free of gammac even at saturating concentrations of IL-4. This quantitative treatment establishes KR as a key measure of the coupling between ligand binding and receptor activation, providing a basis for functional distinctions between different receptors that are activated by ligand-induced receptor dimerization.