Gastrin, or little gastrin, is a heptadecapeptide amide, of molecular weight about 2100, with a single tyrosine residue which may (as in G17 II) or may not be (G17 I) sulphated (Gregory and Tracy 1961, 1964; Gregory et al. 1966; also see Gregory 1969). It can occur as big gastrin (Yalow and Berson 1970, 1971), which has 34 amino acids and also can be isolated in the sulphated or non-sulphated condition, or even as big big gastrin (Yalow and Berson 1972). Berson and Yalow (1971) found the heptadecapeptide (G17-gastrin) to be the major component in extracts of antral mucosa but the proportion of big gastrin (G34) increased distally in the digestive tract, being the sole detectable component in extracts of the mucosa of the proximal jejunum. Both components were immunologically indistinguishable. A ‘mini gastrin’, of perhaps 13 amino acids (G13), equivalent to the 5–17 sequence of little gastrin (G17-gastrin), or to the 21–34 sequence of big gastrin, and capable of stimulating the secretion of gastric acid, has also been reported in tumour tissue (Gregory and Tracy 1974). The different larger forms of the gastrin molecule have been reviewed by Yalow (1974).