Abstract: Anti IL-3 Receptor alpha chain monoclonal antibody (MoAb) is the product of a hybridoma cell line designated 7G3. The MoAb acts as an antagonist to IL-3 in vitro activity. The MoAb binds to the N terminal domain of the IL-3 receptor alpha chain and does so competitively with IL-3 which indicates that this is, at least in part, involved in IL-3 binding. Treatment with the MoAb or fragment thereof, whether recombinant or otherwise, may be suitable for the treatment of one or more of the following conditions: myeloid leukemias, lymphomas such as follicular B cell lymphoma, or the alleviation of allergies.