Asthma is a chronic inflammatory disorder of the airways characterized by reversible airflow obstruction. Fourteen million persons (6.4%) in the United States report having asthma, and from 1980 to 1994 the prevalence of self-reported asthma in the United States increased by 75%. A major factor in the pathogenesis of asthma is the development of an allergic inflammatory response to inhaled antigens. Interleukin-4 (IL-4) plays a key role in this response by promoting IgE production, upregulating IgE receptors, upregulating adhesion receptors such as VCAM-1, promoting Th2 cell development and increasing mucus secretion. Soluble recombinantly produced IL-4R (sIL-4R) has been shown to bind and inactivate IL-4, both in vitro and in animal models. As part of a multicenter trial, 20 subjects at the NIH site will receive 0.9 mg., 1.8 mg. sIL-4R or placebo once weekly for 12 weeks in a double blind placebo controlled study. Study drug will be delivered via the AERx aerosol drug delivery device. The primary objective of the study will be to evaluate efficacy as measured by FEV1. Secondary objectives will include changes in FVC, FEF 27-75, peak flow, bronchodilator usage, asthma symptoms, quality of life scores, immunologic and inflammatory markers, pharmacokinetics, safety and immunogenicity. The study population will consist of moderate to severe asthmatics on (Beta)-agonist monotherapy with an FEV1 of 50-80% of predicted. After 12 weeks of study drug, subjects will be followed for an additional 8 weeks.