We have previously demonstrated that nitroso-chloramphenicol (NO-CAP) in small concentrations causes the hydrolysis of isolated double stranded DNA in vitro and this action is blocked by sulfhydryl groups. The present study was designed to assess damage to isolated DNA as well as intact cells DNA and examine the protective effect of N-acetylcysteine (NAC). Using alkaline sucrose gradient sedimentation and the alkali elution technique of Kohn we were able to demonstrate DNA damage in Raji cells as well as phytohemagglutinin stimulated human lymphocytes after exposure to NO-CAP. Damage could be totally blocked by NAC. In preliminary studies we also observed that NAC protects bone marrow cells from the growth-inhibitory effects of chloramphenicol and thiamphenicol