Gel filtration chromatography of serum from a normal volunteer (top left) and 5 patients with insulinoma (other panels). After gel filtration, insulin immunoreactivity is found in positions corresponding to proteins of 9,000 and 6,000 daltons; immunoreactivity in the 9,000-dalton position is expressed on the basis of both insulin (solid line) and proinsulin (dashed line) standards. The larger protein, appearing nearest the void volume (Vo), contains proinsulin and intermediates of proinsulin conversion. Insulin is indicated by the second peak of immunoreactivity. Proinsulin-like material contributes less than 20%–30% of total immunoreactivity in normal subjects, whereas it is relatively increased in the majority of patients with insulinoma, regardless of total serum insulin concentration (1).