The primary purpose of this protocol is to assess the efficacy, tolerability, and safety of MGA031 when administered according to 3 different MGA031 dosing regimens in children and adults with recent-onset (diagnosis within past 12 weeks) type 1 diabetes mellitus. All regimens will be administered as an addition to insulin and other standard of care treatments. Efficacy will be defined primarily by the capacity of MGA031 to markedly reduce typical insulin requirements while maintaining relatively normal blood sugar levels.