The coding sequence for human poly(ADP-ribose) polymerase was expressed inducibly in Saccharomyces cerevisiae from a low-copy-number plasmid vector. Cell free extracts of induced cells had poly(ADP-ribose) polymerase activity when assayed under standard conditions; activity could not be detected in noninduced cell extracts. Induced cells formed poly(ADP-ribose) in vivo, and levels of these polymers increased when cells were treated with the alkylating agent N-methyl-N'-nitro-N- nitrosoguanidine (MNNG). The cytotoxicity of this agent was increased in induced cells, and in vivo labelling with [3H]adenine further decreased their viability. Increased levels of poly(ADP-ribose) found in cells treated with the alkylating agent were not accompanied by lowering of the NAD concentration.