The hormone gastrin stimulates acid secretion by gastric parietal cells and acts as a growth factor for the gastric mucosa. Gastrin receptors with dissociation constants of approximately 0.5 nm have been detected on isolated gastric parietal cells, and on some cell lines derived from colon carcinomas. We now report that gastrin is also bound by five cell lines derived from human gastric carcinomas, but that the affinities of these lines for gastrin range from 0.2 to 1.3 µm. Cholecystokinin8 binds to the cell line Okajima with an affinity similar to gastrin17, while shorter gastrin analogues bind with reduced affinity. Binding of gastrin is unaffected by acetylcholine, histamine, or a number of other hormones with the exception of insulin which inhibits binding with an IC50 value of 0.5 µm. The ability to bind gastrin with affinities in the µm range appears to be a property widespread among other tumor cell lines.