REIS1–4 has demonstrated the presence of a phosphatase in various tissues specifically hydrolysing 5-nucleotides, for example, adenosine-5-phosphate. One of us has previously reported5 that normal blood serum has a low 5-nucleotidase activity corresponding to about 1/10 of its non-specific alkaline phosphatase activity towards glycerophosphate. In bone diseases where serum alkaline phosphatase is raised, serum 5-nucleotidase may be normal; but in jaundice, either infective or obstructive, both enzyme levels may be increased, those of the 5-nucleotidase sometimes exceeding those of the non-specific alkaline phosphatase.