Insulin analogues labelled with stable isotopes (e.g. deuterium, 18O, 15N, etc.) are authentic (the native structure is rigorously maintained), non-radioactive (preferred for injection into man) and can easily be distinguished from endogenous insulin by mass spectrometry by virtue of their molecular masses. Appropriate combinations of amino-protecting groups (methylsulphonylethyloxycarbonyl and t-butoxy carbonyl), Edman degradation and chemical coupling were used to produce [octadeutero-PheB1]-porcine insulin and [octadeutero-PheB1-octadeutero-ValB2]-porcine insulin. The analogues were characterized by electrospray ionization mass spectrometry. Standard mixtures of labelled and unlabelled insulins were successfully studied by mass spectrometry. Isotope dilution mass spectrometry could therefore provide a useful direct measure of insulin under true physiological conditions, without many of the drawbacks of existing methods. In this regard, the analogue with 16 deuteriums was more suitable than the octadeuterated analogue, since the greater mass difference between the labelled and unlabelled forms enabled a lower mass spectrometric resolution to be used, resulting in higher sensitivity.