Patients with Type 1 diabetes mellitus enrolled in clinical trials at the National Institutes of Health and at the University of Florida and healthy normal volunteers may participate in this study. Participants will have up to three blood samples drawn over a period of less than 1 year. The first sample (about 20 milliliters, or 4 teaspoons), will be examined for changes in the structure of the CD152 gene. If the CD152 structure is different from that normally seen in the general population, a second sample (about 90 ml, or 6 tablespoons of blood) will be drawn. This sample will be used to study the function of specialized immune system cells (T cells), including their growth and survival, chemicals they produce when stimulated, and other factors. If these cells function differently from what is generally seen in the population, a third sample (90 ml) will be drawn for more detailed studies.