To compare disease activity, as measured by PASI score, of three MEDI-507 dosing (injection)regimens vs. placebo.

The primary objective of this study is to compare disease activity, as measured by PASI score, of three MEDI-507 dosing regimens (5 mg for 12 weeks, 5 mg for 6 weeks, 7 mg for 4 weeks) versus placebo administered weekly by SC injection.

Inclusion Criteria:

• Plaque psoriasis involving at least 10% of body surface area
• Age 18 through 65 years at the time of the first dose of study drug
• Both males and females are eligible. However, sexually active females, unless surgically sterile or at least 1 year post-menopausal, must have used an effective method of avoiding pregnancy (including oral or implanted contraceptives, IUD, female condom, diaphragm with spermicide, cervical cap, abstinence, use of a condom by the sexual partner or sterile sexual partner) for 30 days prior to the first dose of study drug and must agree to continue
• Currently receiving no therapy for psoriasis except emollients (certain other over-the-counter products may be allowed with prior approval of the sponsor)
• Written informed consent obtained from the patient
• Ability to complete follow-up period of 167 days as required by the protocol

Exclusion Criteria:

• Pustular, guttate, or erythrodermic psoriasis as the predominant disease type
• PASI score <8
• At screening (must be within 21 days before study entry) any of the following: lymphocyte count under 1,200 cells/mm3, WBC under 4,000 cells/mm3, hematocrit below 32%, platelets below 110,000 cells/mm3, creatinine, AST, ALT over 1.5 times upper limit of normal
• At screening (must be within 21 days before study entry) any clinical evidence of HIV, hepatitis B, hepatitis C or active hepatitis A infection
• Pregnancy (must have a negative serum pregnancy test within 21 days prior to the first dose of study drug, and urine pregnancy test must be negative on Study Day 0 before study entry)
• History of cancer (except excision of basal cell carcinoma)
• Any documented immunodeficiency
• A history of prior administration of monoclonal antibodies or related proteins
• Receipt of systemic retinoids, corticosteroids, cyclosporin A, methotrexate, phototherapy or coal tar treatment in the past 4 weeks
• Use of topical therapy (except emollients) for psoriasis in the past 2 weeks (certain other over-the-counter products may be allowed with prior approval of the sponsor)
• Receipt of any investigational drug therapy within 6 weeks before the first dose of study drug in this protocol (use of licensed agents for indications not listed in the package insert is permitted)
• Current or planned participation in a research protocol in which an investigational agent or therapy may be administered
• Nursing mother
• Acute illness including infections 15. Clinical manifestations of significant end organ dysfunction or failure that may compromise the safety of the volunteer in the study