The intended use of these reagents is for assay of gastrin in human serum. Gastrin in serum is assayed by a competitive radioimmunoassay using a rabbit antiserum raised against a gastrin 17 albumin conjugate. Gastrin in calibrators and samples compete with 125I-labelled gastrin-17 in binding to the antibodies. 125I-gastrin binds in a reverse proportion to the concentration of gastrin in calibrators and samples. Antibody-bound 125I-gastrin is separated from the unbound fraction using the double antibody-polyethyleneglycol precipitation technique. The radioactivity of the precipitates is measured. The antiserum used in this assay cross reacts with gastrin-34 and the sulphated forms of gastrin-17 and gastrin-34.