The minor components of hemoglobin which are increased in subjects with diabetes mellitus, hemoglobin Ala-c, were measured in identical twins concordant and discordant for diabetes to determine whether the observed increases represent a genetically determined abnormality. The mean values for the proportion of hemoglobins Ala-c in discordant twins differed markedly in the two members of the pair: 10.4 +/- 0.74 per cent for the diabetic twins and 6.85 +/- 0.33 for the nondiabetic twins (t = 4.3811, P less than 0.005). In concordant twins with juvenile onset diabetes, the mean proportion of Hb Ala-c was 11.4 per cent, and no marked differences were observed between members of twin pairs. Four pairs of identical twins concordant for maturity onset diabetes showed lower mean values for Hb Ala-c but did not show marked intra-pair differences. Thus, the abnormal proportions of hemoglobins Ala-c found in the presence of overt diabetes mellitus appear to be a manifestation of a metabolic abnormality of diabetes.