Cultured human lymphocytes were exposed to three different concentrations (2.5, 5.0 and 10.0 micrograms/ml) of fluchloralin for 24 and 48 h to assess chromosomal aberrations. A significant dose-dependent increase of chromatid type aberration was observed in these cells. Multiple aberrations (MA) were scored at all concentrations after 48 h treatment. To support these results we did micronucleus (MN) test using cytochalasin B to block cytokinesis. At lower concentrations (2.5 to 10.0 micrograms/ml) the frequency of MN induction was not significantly different. Higher concentrations of fluchloralin (20, 40 and 50 micrograms/ml) resulted in a significant dose dependent increase in number of micronucleated cells. This is the first report on genotoxic effects of fluchloralin in human cells.