It has been proposed that the activation of poly(ADP‐ribose) polymerase (Papirmeister et al., 1985), which results from the presence of strand breaks in bis‐(β‐chloroethyl)sulfide (BCES) damaged DNA, causes depletion in the level of nicotinamide adenine dinucleotide (NAD) leading to cell death. This hypothesis has now been evaluated in the primary submerged culture of rat keratinocytes. The DNA content, the viable cell number, and the proliferative capability (measured by thymidine incorporation) of the culture were all reduced 48 h after exposure to 10 μM BCES. However, the total NAD level, that is, NAD+plus NADH, was not changed at a dose of BCES lower than 50 μM. This observation was the same in both proliferating and early differentiating cultures. To further test this hypothesis, the modifying effect of inhibiting poly(ADP‐ribose) polymerase on cytotoxicity in BCES‐exposed cells was investigated. After exposure to 250 μMBCES, the NAD level was reduced to approximately 26 pmol/μg DNA. This value was increased to 34–49 pmol/μg DNA at both 24 and 48 h postexposure when the cultures were incubated in medium supplemented with 1–10 mM nicotinamide. Nevertheless, the decrease in the DNA content of the culture was not reversed. These results suggest that in the rat keratinocyte culture exposed to BCES, depletion of NAD is not a prerequisite for cell death.