N-Hydroxyparacetamol treatment of lymphoblastoid cells gave rise to a dose-dependent decrease in DNA, RNA, and protein synthesis. Inhibition of DNA synthesis was less marked in medium at pH 6.5 than at pH 9.0. N-Hydroxyparacetamol appeared to inhibit DNA synthesis at least in part through alterations to chromatin structure. This compound produced a dose-dependent and time-dependent loss in the superhelix density of DNA as determined by nucleoid sedimentation analysis. Alkaline elution data as well as sucrose gradient analysis revealed that this decrease in sedimentation did not arise through single strand breakage to DNA. The structural alterations to chromatin caused by N-hydroxyparacetamol appeared to have been repaired after 6 hr. However, sedimentation of "repaired" nucleoids in the presence of ethidium bromide was markedly different from sedimentation of untreated nucleoids. These results suggested that some N-hydroxyparacetamol remained associated with nucleo-protein, thus interfering with the binding of ethidium. Furthermore, both RNA and protein synthesis were markedly inhibited by N-hydroxyparacetamol, demonstrating a major effect on cell function. The widespread effects of N-hydroxyparacetamol could be accounted for by changes to chromatin structure or by a more general effect on cellular metabolism. Either of these effects could account for the dramatic cytotoxicity of this compound. A concentration of 2.5 mM reduced cell viability by 96% after 3 days.