Histamine-releasing factors (HRF) have received much attention over the last 17 years. With the availability of molecular biology techniques, cloned molecules have been identified as HRF. Examples include various interleukins, chemokines and the IgE-dependent HRF This human recombinant HRF (HrHRF) is a novel cytokine with no homology to any known molecule. Indeed, HrHRF acts as a complete secretagogue for basophils possessing a certain type of IgE, namely IgE+. Additionally, HrHRF can enhance histamine release from basophils possessing IgE-, basophils not normally responsive to HrHRF. Furthermore, HrHRF can activate eosinophils from allergic donors. This unique molecule may have a pivotal role in allergic diseases.