AbstractInterleukin-3 (IL-3) regulates the proliferation of myeloid, erythroid, and lymphoid cells. Previous reports showed IL-3 binding restricted to a single high-affinity(Kd= 50-200 pin) site. Here, we demonstrate by equilibrium studies an additional binding site for IL-3 with lower apparent affinity(Kd=5-20 iym).Furthermore, kinetic analysis shows that two binding sites for IL-3 exist: IL-3 dissociates slowly from the first site (I½=4hr; k−1=2.7x10−3min−1), whereas it dissociates rapidly (T½=4.0 min; k−1= 0.116 min−1) from the second site. Cross-linking showed that [125I]IL-3 binding to the 115- and 140-kD proteins was not saturable at concentrations commensurate with high-affinity binding and IL-3 dissociated rapidly from these same molecules. Thus, the low affinity IL-3 receptor is a molecule(s) of 115- to 140-kD.