Incubation of cells with thymidine (dThd) is known to cause dNTP pool imbalance as well as deletions and depletion of the mtDNA. In order to gain further understanding in the events involved in dThd toxicity over time, H9 cells were cultured for 20 months in the presence or absence of 1 micro M dThd. The level of mtDNA was reduced by 90% in the cells grown in dThd as compared to the control cells. The H9/dThd cells also showed a 100-fold increased sensitivity towards the cytotoxicity of the antileukemic compound 9-beta-D-arabinofuranolsylguanine (araG).