In addition to the previously described molecular forms of gastrin, a new component has been found in high concentrations in the serum of 6 patients with Zollinger-Ellison syndrome. Serum was fractionated by gel filtration, and ion exchange chromatography, and the new component was identified in eluates by radioimmunoassay using an antibody with specificity for the N-terminal portion of heptadecapeptide gastrin. The precise chemical nature of the new component is not known, but its chromatographic behavior and its reactivity to various antibodies is indistinguishable from that of the natural or synthetic N-terminal 1 to 13 fragment of G-17. The new component is present in gastrinoma tumor tissue. Its concentration in serum of gastrinoma patients increases markedly when secretin is injected.