In human erythrocyte membranes incubated with [adenylate-32P]NAD the 36 kDa protein is predominantly labeled. The labeling is greatly stimulated by nitroprusside in the presence of dithiothreitol. We have purified the 36 kDa protein and identified this modification as cysteine-specific mono(ADP-ribosylation) because: (i) labeling occurred only when [32P]NAD was replaced by adenine[U-14C]NAD, but not by [carbonyl-14C]NAD; (ii) treatment of the prelabeled protein with snake venom phosphodiesterase led to releasing 5'-[32P]AMP; (iii) the bond between the protein and the nucleotide was hydrolyzed by HgCl2, but was resistant to hydroxylamine. The 36 kDa protein reacted on Western blots with two different monoclonal antibodies (MAbs) against glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and was immunoprecipitated by both MAbs.