To evaluate the safety of treating subjects with up to 12 additional doses of alefacept.

Male and female subjects at least 18 years of age with moderate to severe plaque psoriasis treated with 12 weeks of alefacept 15 mg IM and who have not achieved the desired response.

Dosing Groups: Subjects will receive either 4, 8, or 12 doses of alefacept 15 mg IM weekly immediately (within 14 days) following a standard 12-dose course of AMEVIVE® 15 mg IM. Determination of number of doses will be based on physician qualitative assessment at weeks 4 and 8.

Inclusion Criteria:

1. Must give written informed consent.
2. Must have had moderate, moderately severe or severe chronic plaque psoriasis as determined by the investigator prior to initial treatment (baseline) with AMEVIVE.
3. Must be 18 years of age or older.
4. Must have completed a standard 12-week course of AMEVIVE and have received at least 10 doses.
5. Response to current AMEVIVE therapy must be less than a desired response as determined by the physician, and subject and some residual psoriasis must be present.

Exclusion Criteria:

1. Female subjects who are not postmenopausal for at least 1 year, surgically sterile, or not willing to practice effective contraception during the study.
2. Nursing mothers, pregnant women, and women planning to become pregnant
3. Current enrollment in any investigational study in which the subject is receiving any type of drug, biologic, or non-drug therapy.
4. Treatment with another investigational drug, or approved therapy for investigational use, within 3 months of investigational drug administration.
5. Treatment with systemic retinoids, systemic steroids, methotrexate, cyclosporine, azathioprine, thioguanine, etanercept, efalizumab, infliximab, adalimumab or mycophenolate mofetil or other systemic immunosuppressant agents within 4 weeks of investigational drug administration.
6. Treatment with Ultraviolet B (UVB) phototherapy or Psoralen + Ultraviolet A (PUVA), within 4 weeks of investigational drug administration.
7. Serious local infection (e.g., cellulitis, abscess) or systemic infection (e.g., pneumonia, septicemia) within the 3 months prior to the first dose of investigational drug.
8. History of >3 cutaneous squamous cell carcinomas or any systemic malignancy.
9. Skin lesions suspicious for malignancy.
10. Known HIV, viral hepatitis, or tuberculosis infection.
11. History of severe allergic or anaphylactic reactions.
12. ALT or AST greater than three times the upper limit of normal.
13. Significantly abnormal hematology (hemoglobin, hematocrit, platelets, white blood cells), as determined by the investigator.
14. CD4+ T lymphocyte count at screening visit less than 250 cells/mm3.
15. Known hypersensitivity to AMEVIVE or any of its components.
16. Subject's inability to comply with study requirements.