The in vitro mammalian metabolism of the fungicide zoxamide is related to its in vitro mammalian toxicity. After incubation of zoxamide with rat liver microsomes leading to practically 100% metabolism (mostly hydroxylated zoxamide), the cytotoxicity (methyl thiazole tetrazolium (MTT) test) and the mitosis-inhibiting potential (shown by cell count and by cell cycle analysis) for V79 were not distinguishable from those of zoxamide, demonstrating that the hydroxylation of zoxamide did not change the cytotoxicity or mitosis-inhibiting potential as determined by these assays. After incubation of zoxamide with rat liver S9 predominantly leading to conjugation with glutathione, and after incubation of zoxamide with rat liver slices predominantly leading to the glucuronide of the hydroxylated zoxamide, these activities were eliminated demonstrating that the glutathione conjugate and the glucuronide had lost the activities in these assays due either to no intrinsic potential of these conjugates or to their inability to penetrate the plasma membrane of mammalian cells. It is concluded that the metabolic hydroxylation of zoxamide did not change its activity in the assays used for investigating its influence on cell proliferation, cell cycle and cytotoxicity, while the formation of conjugates with glutathione or glucuronic acid led to the apparent loss of these activities. Thus, with zoxamide as a prototype, it was shown that, in principle, mammalian metabolism and its relationship to mammalian detoxication of fungicidal mitosis inhibitors may be reasonably anticipated from in vitro studies. In addition, the results provide a rational for the observed absence of typically mitosis inhibition-associated toxicities of zoxamide in mammals in vivo.