Significantly decreased activity of pancreatic isoamylase in serum was found in a group of 51 juvenile-onset insulin-dependent diabetics as compared to healthy subjects (p less than 0.005). No significant changes were observed for urinary p-aminobenzoic acid excretion in 20 of the juvenile-onset diabetics in whom the NBT-PABA test was performed, even though 25% of the values were below the normal limit. A highly significant decrease of serum lipase activity was found in juvenile-onset diabetics as compared to controls (p less than 0.001). No significant correlation was found in juvenile-onset diabetics between serum pancreatic isoamylase and lipase or marker of chymotrypsin activity expressed as the amount of p-aminobenzoic acid excreted into urine. The NBT-PABA test appears to be of small importance in the evaluation of changes of the exocrine pancreas in insulin-dependent diabetes mellitus. However, simultaneous evaluation of serum pancreatic isoamylase and lipase activities justified the suspicion of pancreatic damage in 50% of the patients tested.