The development of insulin-dependent diabetes mellitus (IDDM) includes a prodrome of autoimmunity against pancreatic beta cells. The period of subclinical islet cell disease with altered beta-cell function may be prolonged. We have determined the serum pancreatic alpha-isoamylase in both young diabetes-prone (DP) and newly diabetic BB rats to test whether changes in the pancreas prior to IDDM are reflected by this enzyme, shown to be regulated by insulin. A prospective analysis of inbred BB rats (n = 28) that later developed diabetes showed that the alpha-isoamylase at the time of onset was reduced by 19% (p less than 0.02) compared with levels observed 1 week earlier and by 30% (p less than 0.01) compared with levels 2 weeks before onset. Furthermore, when compared to age-matched diabetes-resistant (DR) BB rats in a cross-sectional study, the DP BB rats investigated in groups at 20, 30, 40, 50, 60, and 70 days of age had significantly lower (p less than 0.01) serum alpha-isoamylase already from 50 days of age, which is 2-6 weeks prior to the expected onset of diabetes. Finally, in 70-day-old cofostered DP and DR male rats with identical body weight and rates of growth, the serum alpha-isoamylase was decreased in the DP yet nondiabetic (n = 8) rats compared with the DR (n = 8) rats (p less than 0.05). Reduced levels of serum alpha-isoamylase, therefore, may reflect loss of beta cells or beta-cell function in the pancreas of diabetes-prone but not yet diabetic BB rats.