The activity of Adenosine diphosphoribosyl transferase (ADPRT), the chromatin-bound enzyme which specifically catalyses the cleavage of oxidized NAD⁺ with the concomitant covalent attachment of the ADP-ribose moiety to acceptor proteins, was investigated in Polyoma Virus-Transformed (PyY) and Untransformed BHK21/C13 (BHK) cells. It was shown that ADPRT activity was consistently 2-4 fold higher in PyY cells than in BHK cells. The spectrum of (ADP-ribose)_n residues synthesised by the two cell types was very similar when analysed by hydroxyapatite column chromatography. Poly (ADP-ribose) Glycohydrolase activity in the two cell types was identical with 25-30% degradation of the poly(ADP-ribose) over a period of 90 minutes. DNA damage resulting from incubation with Deoxyribonuclease was reflected by an immediate increase in ADPRT activity and an increase in (ADP-ribose)_n chain length by both cell types. Polyamines which are present at high concentrations in rapidly dividing tissues were able to stimulate ADPRT activity both <i>in vitro</i> and <i>in vivo</i> in BHK and PyY cells. In general the average chain length of ADP-ribose residues synthesised remained unaltered. No significant increase in the level of DNA-strand breakage could be detected in the polyamine-treated cells. Depletion of the cellular polyamine levels resulted in stimulation of ADPRT activity, but there was no significant difference in the spectrum of (ADP-ribose)_n residues synthesised. Again no significant increase in the level of DNA-strand breaks could be detected in the polyamine-depleted cells. These results suggest that DNA-damage may not be the only means of regulating ADPRT activity and that polyamines may have a role to play in this regard <i>in vivo</i>.