Auto-ADP-ribosylation of arginine-specific ADP-ribosyltransferase purified from chicken peripheral heterophils was investigated. When the purified ADP-ribosyltransferase was analyzed with sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by Coomassie brilliant blue staining, two protein bands corresponding to the molecular masses of 27.5 and 28.0 kDa were detected. Both proteins were auto-ADP-ribosylated when they were examined by zymographic in situ gel assay without exogenous acceptor for ADP-ribose transfer. The automodification was inhibited by the acceptor, arginine or agmatine, and an inhibitor of arginine-specific ADP-ribosyltransferase, novobiocin. The ADP-ribose-transferase linkage was labile in 0.5 M hydroxylamine (pH 7.5). The automodified transferase was not chased by a large excess of nonradioactive NAD and did not catalyze transfer of its ADP-ribose to p33, an endogenous substrate protein for the transferase in heterophils, therefore, that automodified transferase cannot serve as an intermediate in ADP-ribosylation of other proteins. Auto-ADP-ribosylated transferase showed higher activity than did the unmodified transferase in catalyzing ADP-ribosylation of the basic acceptor such as poly(L-arginine) and p33 while to ADP-ribosylate the acidic proteins such as casein, the modified transferase was less active. Automodification of the transferase decreased polyanion-induced ADP-ribosylation of p33. Automodification of arginine-specific ADP-ribosyltransferase apparently alters the specificity of its own substrate.