Nucleotide pyrophosphatase activity with uridine diphosphoglucose and dephospho-CoA as substrates was demonstrated in normal human serum. The enzyme has a pH-optimum of about 9.6 and is inhibited by EDTA. Phosphodiesterase I (hydrolysis of thymidine-5'-monophospho-p-nitrophenylester) was also found in normal human serum, with a pH-optimum of about 9.8 and a Km of 0.20-0.25 mM. Probably both activities should be attributed to one enzyme. Different isoenzymes may exist, however. The activity of nucleotide pyrophosphatase/phosphodiesterase I in normal serum in many respects resembles an enzyme previously isolated from liver plasma membranes. Phosphodiesterase I activity was increased in normal pregnancy, in primary biliary cirrhosis, and in patients with bone lesions, but not in acute viral hepatitis or active chronic hepatitis. In primary biliary cirrhosis, the activity of phosphodiesterase I paralleled an increase of alkaline phosphatases.