Abstract: A radioiodinated phloretyl-.beta.-alanyl-secretin, its salts and its protected derivatives. A process for the manufacture of a radioiodinated phloretyl-.beta.-alanyl-secretin and/or a salt thereof which process involves reacting phloretyl-.beta.-alanyl-secretin and/or a salt thereof with radioactive iodine. Phloretyl-.alpha.-alanyl-secretin, its salts and its protected derivatives. A method for the determination of secretin in a sample, which involves; mixing the sample with a known amount of a radioiodinated phloretyl-.beta.-alanyl-secretin and an antibody specific for secretin; measuring the degree of binding of the labelled radioiodinated phloretyl-.beta.-alanyl-secretin; and determining the amount of secretin present in the sample by comparing the degree of binding with a standard curve which has been obtained by adding known amounts of secretin to a defined mixture of the radioiodinated phloretyl-.beta.