A therapeutic agent capable of of binding to the F'-G' loop of domain 4 of the common .sub.c Chain of the receptor for GM-CSF, IL-3, and IL-5. The amino acid Tyr.sup.421 which is located in the F'-G' loop is critical in the high affinity binding and stimulation of function of GM-CSF, IL-3 and IL-5 to the common .sub.c chain of the receptors. Other receptors for cytokines also show hydrophobic amino acids in analogous positions and it is probable that they too play a pivotal role in binding of respective cytokines to them and in modulating function. Agents capable of binding to the F'-G' loops are suggested and should be of therapeutic value.