Male or nonpregnant, non-breastfeeding females age 12-85 years.

Diagnosis of persistent asthma for greater than 1 year and currently being treated with short acting beta-2 agonist only.

FEV1 50-80% of predicted (must be demonstrated at Day 10 and Day 0). Patients will be stratified into 50-70% or 71-80% cohort at Day 10.

Increase of greater than or equal to 15% over baseline FEV1 approximately 15-20 minutes after beta-2 agonist inhalation (2-4 puffs of albuterol via MDI or the nebulized equivalent) documented at baseline.

Positive prick skin test to at least two allergens (defined as wheal 3 mm greater than control and erythema greater than control).

History of asthma symptoms (wheezing, shortness of breath, cough, chest tightness, or nighttime awakening) on least 3 of the last 7 days.

Fulfillment of washout criteria by not using any of the medications listed below for the specified times prior to Day 0 of the study drug treatment:

Parenteral corticosteroids for 4 weeks;

Oral corticosteroids for 4 weeks;

Inhaled corticosteroids for 4 weeks;

Cromolyn sodium (Intal), nedocromil (Tilade) for 4 weeks;

Theophylline, zileuton (Zyflo), zafirlukast (Accolate), or montelukast (Singulair) for 4 weeks;

Astemizole (Hismanal) for 12 weeks;

Terfenadine (Seldane), or fexofenadine (Allegra) for 6 days;

Cetirizine (Zyrtec) for 6 days;

Hydroxyzine (Atarax, Vistaril) for 6 days;

Azelastine (Astelin) nasal spray for 6 days; and

Salmeterol (Serevent) for 9 days.

No clinically significant abnormality in chemistry, hematology, urinalysis: serum creatinine less than or equal to 1.7 mg/dL; total bilirubin less than or equal to 1.5 mg/dL; AST (SGOT), ALT (SGPT) less than or equal to 2 times laboratory's upper limit of normal.

No clinically significant abnormality in EKG within 1 month prior to enrollment.

No clinically significant abnormality in CXR (other than changes consistent with asthma) within 1 year prior to enrollment.

Nonsmoker, for at least 2 years with a smoking history of no more than 10 pack years (e.g., one pack per day for 10 years).

Agreement to use medically accepted contraception throughout the study, if sexually active, except females who are postmenopausal for greater than or equal to 2 years.

Agreement not to donate blood or blood products throughout the study.

Demonstrated ability to follow proper technique in the use of the AERx system.

A written, signed, and witnessed consent form.

No desensitization therapy within 3 months prior to Day 0 of study drug treatment.

No use of any investigational or non-approved drug therapy within 30 days prior to Day 0 of study drug treatment.

No occurrence of acute asthma exacerbation requiring emergency room treatment within 6 weeks of Day 0 of study drug treatment.

No occurrence of acute asthma exacerbation requiring hospitalization within 12 months of Day 0 of study drug treatment.

No occurrence of respiratory infection which affects asthma within 4 weeks prior to Day 0 of study drug treatment.

No history of endotracheal intubation for asthma-related exacerbation within 15 years prior to Day 0 of study drug treatment.

No presence of significant medical conditions (including obesity affecting respiratory function, congestive heart failure, myocardial infarction, unstable angina, uncontrolled hypertension, severe pulmonary disease, history of cancer [other than resected cutaneous basal or squamous cell carcinoma], insulin-dependent diabetes, autoimmune disease, or known HIV infection).

No previous enrollment in a study of soluble IL-4 receptor.

No history of alcohol abuse, drug abuse, or psychiatric illness that would interfere with ability to comply with protocol requirements or give informed consent.

No patients experiencing hypersensitivity to soluble IL-4R.