Dextro (d) and laevo (l) forms of quartz dust was inoculated intranasally and subcutaneously into 2-month-old (C57 x BALB/c)F1 mice which were killed 18 months later. Both d and l quartz caused fibrosis, liver nodule formation and an enhanced incidence of lymphoma leukemia. However, the tumour incidence was significantly higher in l than in d quartz dust-treated animals. It is concluded that in addition to effects clearly independent of chirality there is evidence that the biological response to nonidentical mirror images of solid crystals may differ.