Conflicting reports exist on the postprandial response of serum cationic trypsin like immunoreactivity (SCTLI). The purpose of the current study was to examine the postprandial response of SCTLI in healthy persons and in insulin dependent diabetes mellitus (IDDM) patients and relate this to C-peptide secretion. The study was performed on six healthy males 25-40 years old and in nine male IDDM patients 25-38 years old. To all the patients after an overnight fast was given a standardized breakfast. Blood samples were obtained through a butterfly needle inserted into the antecubital vein at 0, 30, 60 and 120 minutes. Radioimmunoassays were performed using commercially available kits: for SCTLI CIS-SORIN and for C-peptide BIODATA. In healthy persons there was a significant rise of SCTLI concentration at 30 minutes, and C-peptide at 30 and 60 minutes. IDDM patients showed lower basal levels of SCTLI and C-peptide, without any significant changes of SCTLI and C-peptide during the test. We concluded that trypsin appearance in serum closely correlates with endocrine pancreatic function.