Caffeine is known to potentiate the cytotoxicity of a variety of DNA damaging agents presumably by reducing the ability of the cells to repair potentially lethal lesions. However, in the present study we observe that 5 mM caffeine reverses the cell kinetic and cytotoxic effects of the intercalating drug Novantrone (mitoxantrone) on L1210, HL-60 and CHO cells. Novantrone alone, at a concentration of 20-30 ng/ml, given to cultures for 1 h, inhibits cell growth by about 50% and causes cells to accumulate in S and G2 phase and to enter a higher DNA ploidy level. Treatment of these cell lines with 5 mM caffeine alone for 1 h has a minimal effect on cell proliferation; suppression of cell growth varies from 5 to 10%. Exposure of cells to Novantrone for 1 h in the presence of caffeine results in a significant reduction of the Novantrone effects; the cell growth rate is partially restored (e.g. caffeine reduces suppression of L1210 cell growth from 48 to 83% of control) and in each of the cell lines studied, the Novantrone-induced cell accumulation in S and G2 is abolished. Combined treatment with caffeine and Novantrone also increases the clonogenicity of CHO cells 8.5 times over that seen in cultures treated with Novantrone alone. In contrast to the combined treatment with caffeine + Novantrone, pretreatment of cells with caffeine provides no protection. Likewise, post-treatment with caffeine provides little reversal of growth inhibition and G2 cell accumulation, especially if the post-treatment is delayed in time. The present data, in conjunction with evidence in the literature that caffeine protects cells against the cytotoxic effects of doxorubicin, suggest that caffeine may play a more general role in protecting cells against planar aromatic molecules such as intercalating agents.