Abstract Poly(ADP-ribose) synthetase from bovine thymus was phosphorylated effectively by protein kinase C in vitro . The phosphorylation was dependent on the activators of this kinase, Ca 2+ and phospholipid. The apparent Km for the synthetase was about 8 μM, which was lower than that for histone H 1. Though the synthetase was a weak substrate for Ca 2+/calmodulin-dependent protein kinase II, other protein kinases, cyclic AMP-dependent and cofactor-independent protein kinases did not phosphorylate the synthetase. Phosphorylation of the synthetase by protein kinase C resulted in appreciable inhibition of the synthetase activity.