This trial is conducted in Europe. The aim of the trial is to test for bioequivalence between two formulations of insulin degludec/insulin aspart (IDegAsp) to see if the efficacy and safety obtained with formulation 1 can be assumed identical for formulation 2.

Inclusion Criteria:

• Type 1 diabetes mellitus (as diagnosed clinically) for 12 months or longer
• Body mass index from 18.0 to 28.0 kg/m^2 (both inclusive)
• HbA1c below or equal to 9.5% by central laboratory analysis

Exclusion Criteria:

• Subject who has donated any blood or plasma in the past month or more than 500 mL within 3 months prior to screening
• Smoker (defined as a subject who is smoking more than 5 cigarettes or the equivalent per day)
• Not able or willing to refrain from smoking and use of nicotine gum or transdermal nicotine patches during the inpatient period