This study will assess the efficacy, safety and tolerability of two doses of ACT 128800 in patients with moderate-to-severe chronic plaque psoriasis.

Inclusion Criteria:

• Adult males and females aged 18 to 60 years (inclusive) with moderate to severe chronic plaque psoriasis who require systemic treatment.

Exclusion Criteria:

• Patients with other forms of psoriasis and patients who are currently treated for autoimmune disorders other than psoriasis.
• Systemic or topical treatments for psoriasis other than emollients. Ongoing bacterial, viral or fungal infections.

History or presence of malignancy.

• Additional inclusion and exclusion criteria apply with respect to medical conditions and concomitant treatments which could affect patients' risk from participating in the study.