The purpose of this study is to determine the safety and efficacy of 2 doses of ABT-874 versus placebo in the treatment of subjects with moderate to severe plaque psoriasis.

Inclusion Criteria:

• Subject is >= 18 years of age
• Subject had a clinical diagnosis of psoriasis for at least 6 months, and had moderate to severe plaque psoriasis
• Subject is judged to be in generally good health as determined by the principal investigator

Exclusion Criteria:

• Subject has previous exposure to systemic anti-IL 12 therapy
• Subject cannot discontinue systemic therapies and/or topical therapies for the treatment of psoriasis and cannot avoid UVB or PUVA phototherapy
• Subject is taking or requires oral or injectible corticosteroids
• Subject diagnosed with erythrodermic psoriasis, pustular psoriasis, medication-induced or medication-exacerbated psoriasis or new onset guttate psoriasis
• Subject considered by the investigator, for any reason, to be an unsuitable candidate
• Female subject who is pregnant or breast-feeding or considering becoming pregnant