Poly(ADP-ribosylation) was demonstrated in the intestinal parasite Ascaris suum, especially in the reproductive tissues. The activity of the ADP-ribosyltransferase was found to depend on divalent cations and to be stimulated by deoxyribonuclease I about 5-fold. The reaction rate was optimal at a temperature of 30 degrees C and at pH about 8.4. The apparent Km value for NAD was estimated to be 0.2mM. The enzyme activity was effectively inhibited by nicotinamide (Ki = 65 microM) benzamide (6 microM), 3-aminobenzamide (10 microM), theophylline (35 microM) and thymidine (50 microM). The type of inhibition by these compounds was found to be competitive with respect to NAD.