Cysteamine determination is performed by using a coupled enzymatic system. The reaction product between cysteamine and bromopyruvate inhibits competitively and specifically D-amino acid oxidase activity. A reaction system allowing the quantitative assay of cysteamine at the nanomole level has been worked out. The standard curve is linear and the assay is useful in determining the cysteamine concentration in tissues and biological samples. The method is fast and of simple execution. The cysteamine concentration in several mammalian organs is also reported.