The study design is a double blind placebo controlled trial that will enroll subjects between the ages of 8 - 30 who have had the diagnosis of Type 1 diabetes made 4 - 12 months prior to enrollment. A single 14 course of treatment with mAb hOKT3gamma1(Ala-Ala), Teplizumab will be given. The primary endpoint is the C-peptide response to a mixed meal at 12 months. A total of 60 subjects will be enrolled (30 in the drug treatment and 30 in the placebo groups) at 4 study sites: Yale University,the University of California at San Francisco, Children's Hospital of Philadelphia, and the Barbara Davis Diabetes Center.