Incubation of human serum samples with [adenine-14C]NAD resulted in a time- and dose-dependent incorporation of adenine moiety into CCI3COOH-precipitable material. Incorporated radioactivity was relatively resistant to neutral hydroxylamine, but was completely released by treatment with NaOH. An incorporation was observed also after preincubation of NAD with NAD glycohydrolase from pig brain. NAD glycohydrolase activity in serum samples was then shown spectroscopically in an assay coupled to alcohol oxidation. Thus, this reaction was implicated to be due to the binding of ADP-ribose, formed under the action of a soluble, endogenous NAD glycohydrolase activity, to serum proteins. Analysis by NaDodSO4/polyacrylamide gel electrophoresis (PAGE) and autoradiography indicated that a polypeptide of 97 kD, but also two further polypeptides of higher molecular weight and serum albumin, were labelled after incubation with radioactive NAD.