A method of isolating a monoclonal antibody capable of inhibiting any one of IL-3, GM-CSF and IL-5 binding to the common receptor .beta..sub.c, or a monoclonal antibody capable of inhibiting the cytokines binding to a receptor analogous to .beta..sub.c. The method includes the steps of immunizing an animal with a cytokine receptor or portion of a cytokine containing the critical binding site which portion includes the extracellular domain 4 or analogous domain in the analogous common receptor or part thereof. Antibodies producing cells from the animal are then isolated and fused with a myeloma cell line and then screened for a cell line that produces an antibody of the desired type. A monoclonal antibody, or fragments thereof capable of inhibiting the binding of the cytokines IL-3, GM-CSF and IL-5 to the .beta..sub.c receptor, and a hybridoma cell line producing the antibody are also claimed.