Inclusion Criteria:

Subjects must meet all of the following criteria:

1. Enrollment (Segment #1) or randomization (Segment #2) on Study Day 0 within 12 weeks of first visit to any physician for symptoms or signs of diabetes. Study Day 0 is the first day of study drug dosing.
2. Diagnosis of type 1 diabetes mellitus, according to the American Diabetes Association (ADA) criteria
3. Requirement for injected insulin therapy
4. Have a detectable fasting or stimulated C-peptide level (above the lower limit of detection of the assay)

5. One positive result on testing for any of the following antibodies:

   1. islet-cell autoantibodies (ICA512/IA-2),
   2. glutamic acid decarboxylase autoantibodies, or
   3. insulin autoantibodies (if present during first 2 weeks, but not beyond 2 weeks, of insulin treatment)
6. Male or female

7. Subject must be in one of the following age groups:

   • Age 18-35 years
   • Age 12-17 years pending approval by Data Monitoring Committee
   • Age 8-11 years pending approval by Data Monitoring Committee
8. Body weight ≥ 36 kg

Exclusion Criteria:

Subjects must have none of the following:

1. Prior administration of a monoclonal antibody -- within the 1 year before enrollment or randomization at Study Day 0 -- that could potentially prevent or confound a therapeutic response to MGA031
2. Participation in any type of therapeutic drug or vaccine clinical trial within the 12 weeks before enrollment or randomization
3. Any medical condition that, in the opinion of the investigator, would interfere with safe completion of the trial
4. Pregnant or lactating females
5. Prior murine OKT®3 treatment at any time before enrollment or randomization
6. Current or planned therapy with exenatide or any other agents that stimulate pancreatic beta cell regeneration or insulin secretion
7. Current or planned therapy with inhaled insulin
8. Uncompensated heart failure, fluid overload, myocardial infarction or evidence of ischemic heart disease, or other serious cardiac disease within the 12 weeks before enrollment or randomization
9. History of epilepsy, cancer, cystic fibrosis, sickle cell anemia, neuropathy, peripheral vascular disease or cerebrovascular disease
10. Newly diagnosed hypothyroidism (not currently being treated but which, in the opinion of the investigator, should be treated) or active Graves' disease
11. Eczema, asthma or severe atopic disease requiring treatment within the 12 weeks before enrollment or randomization
12. Evidence of active infection, such as fever ≥ 38.0 degrees Celsius (100.5 degrees Fahrenheit)
13. Known or suspected infection with human immunodeficiency virus (HIV)
14. Evidence of active hepatitis B (HBV) or hepatitis C virus (HCV)
15. Evidence of active or latent tuberculosis
16. Vaccination with a live virus within the 12 weeks before enrollment or randomization or planned live virus vaccination continuing through week 52 of the study. Vaccination with an antigen or killed organism must not be given within 12 weeks before or planned within 8 weeks after each dosing cycle.
17. Any infectious mononucleosis-like illness within the 6 months before enrollment or randomization
18. Serologic and clinical evidence of acute infection with Epstein-Barr virus (EBV)
19. Serologic evidence of acute infection with cytomegalovirus (CMV)