Metabolism of two broad spectrum quinolones in advanced preclinical development-tosufloxacin and BMY 43748-was compared in hepatocyte cultures derived from rat, dog, monkey and humans. The drugs were added at the first medium renewal and incubated for 4 or 24 hr in serum-free medium. Metabolites were analysed by HPLC. Marked species differences were found in both the metabolic rates and pathways of the two quinolones. BMY 43748 was found to be more actively metabolized than tosufloxacin in cultured hepatocytes from the three animal species. Human hepatocytes were totally ineffective, thereby indicating that they were metabolically different from monkey cells. Comparison with available in vivo data demonstrated a good but not perfect qualitative in vivo/in vitro correlation.