Gastric inhibitory polypeptide (GIP) is released from the duodenum and jejunum following the ingestion of glucose, fat and amino acids. This hormone potentiates the glucose-induced insulin release from the beta-cells of the pancreas. The role of GIP as "incretin" is discussed. The method of the radioimmunoassay for the determination of GIP in serum samples is described. The lower limit of sensitivity of the GIP radioimmunoassay is in the range of 30-50 pg per ml serum. The described radioimmunoassay is sensitive enough to determine fasting levels of GIP in normal subjects (287 +/- 59 pg/ml). The clinical and pathophysiological importance of GIP is discussed by means of various diseases (obesity, maturity-onset diabetes mellitus, duodenal ulcer disease).