Serum insulin was purified by immunoaffinity chromatography and was then subjected to HPLC, and immunoreactive insulin was measured in all the fractions eluting from the HPLC column. The amount of immunoreactive insulin is plotted against HPLC fraction number in each of the profiles. Arabic numerals identify the individual with respect to subsequent experiments; subject 3 is the propositus. Roman numerals I–IV identify the elution positions of human [Ser^B24]-insulin, bovine insulin, normal human insulin, and porcine insulin, respectively. Columns to the right of the figure list the fasting serum levels of immunoreactive insulin and glucose determined for each of the seven subjects.