Potentially eligible subjects with Type 1 diabetes mellitus who are treated with with a basal-bolus insulin regimen through multiple daily injections or insulin pump at a total daily insulin dose ≤60 U, will be eligible. Visit 1 is approximately 3-6 weeks prior to randomization. Given some variability in HbA1c and C-peptide assays, re-testing for HbA1c and C-peptide can be performed within 18 days from the initial visit. Visit 2 is approximately 2-5 weeks prior to randomization. Subjects are on lispro insulin throughout study except during Visit 4 and Visit 5, the domicile 24 hr treatment period, when they are switched to regular insulin U-100.