Digestion of the poly(ADP-ribose) with snake venom phosphodiesterase yielded phosphoribosyl-AMP and AMP in a ratio of 30 to 1, indicating the "chain length" of 30 if the poly(ADP-ribose) is a linear molecule as previously supposed. Gel filtration and gel electrophoresis, however, revealed a very heterogeneous size distribution, the average size being, much longer than (ADP-ribose)30. Electron microscopy of poly(ADP-ribose) showed an irregularly branched figure. Molecules with more than ten branched portions were observed. These results are in agreement with the recent identification of the chemical structure of the branched portion in poly(ADP-ribose).