The purpose of this study is to evaluate the safety, tolerability, pharmacodynamics (PD) and pharmacokinetics (PK) of subcutaneously administered teplizumab in adults with relatively recent onset T1DM, greater than 12 weeks and less than 52 weeks of presentation of first signs and symptoms of disease to a physician prior to randomization.

Inclusion Criteria:

• Randomization after 12 weeks but within 12 months from first doctor visit for symptoms or signs of diabetes
• Diagnosis of type 1 diabetes mellitus
• Currently receiving insulin therapy
• Detectable fasting or stimulated C-peptide level at screening
• One positive autoantibody test result at screening: Islet-cell autoantibodies (ICA512)/islet antigen-2 (IA-2),Glutamic acid decarboxylase (GAD) autoantibodies, or Insulin autoantibodies

Exclusion Criteria:

• Any medical condition that, in the opinion of the investigator, would interfere with safe completion of the trial
• Previous treatment with monoclonal antibody
• Current treatment with oral antidiabetic agents
• Evidence of active infection