This trial is conducted in Europe. The aim of the trial is to investigate the pharmacodynamic (the effect of the investigated drug on the body) and the pharmacokinetic (exposure of the trial drug in the body) properties of insulin degludec/insulin aspart 15 in subjects with type 1 diabetes.

Inclusion Criteria:

• Type 1 diabetes mellitus (as diagnosed clinically) for at least 12 months
• Body mass index 18.0-28.0 kg/m^2 (both inclusive)

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, use of nicotine substitute products or transdermal nicotine patches during the inpatient period
• Female of childbearing potential who is pregnant, breast-feeding or intend to become pregnant or is not using adequate contraceptive methods (adequate contraceptive measures include sterilisation, hormonal intrauterine devices, oral contraceptives, condom with spermizide, sexual abstinence or vasectomised partner)