4. Subjects presenting with active FCAS / MWS based on clinical signs/symptoms in addition to elevated acute phase reactants (CRP, SAA or ESR). To meet established clinical criteria for active disease, patients must have recurrent intermittent episodes of fever and rash, an age of onset < 6 months of age, duration of most attacks < 24 hours, and the presence of conjunctivitis with attacks. Active disease will be defined as either the presence of classical features or a history of such features that became quiescent in the setting of therapy with anakinra or rilonacept. However, before a patient who has quiescent disease and is currently taking anakinra or rilonacept can receive study drug, he/she must fulfill criteria for active disease after anakinra or rilonacept has been discontinued. Subjects must be greater than 48 hours from their last dose of anakinra (half-life 4-6 hours) and 14 days from their last dose of rilonacept (half-life 7.5 days) before beginning XOMA 052 therapy, and will not take anakinra or rilonacept for the remainder of their enrollment in the study.