Cytokines are potent regulatory molecules involved in cell to cell communication. They have a clear role in the development of inflammation that is seen in both asthma and occupational lung diseases. The eosinophilia that is characteristic in the airways in asthma, is predominantly regulated by the cytokine, interleukin 5. It is thought to be such an important mediator that attempts have been made to develop drugs to alter it's function. In many of the occupational lung diseases however, the major inflammatory cell is the neutrophil. Here, it is the cytokine, interleukin 8 that is believed to influence neutrophil accumulation.