The mitotic cell-cycle duration of root meristematic cells of Eichhornia crassipes as determined by the colchicine labelling method was approximately 24 h at 30 +/- 1 degrees C. In one experiment the intact root meristems of E. crassipes were subjected to 1 h acute exposure to water contaminated with maleic hydrazide (MH), 56 ppm, or methyl mercuric chloride (MMCl), 0.1-0.5 ppm, followed by recovery in tap water for 4-48 h. In a second experiment the roots were subjected to 96 h exposure to water contaminated with MH, 56 ppm, or MMCl, 0.0001-0.1 ppm. In both experiments the cytological end-point measured was the frequency of cells with micronuclei (MNC). In the first experiment, while in the MH-exposed root meristems the frequency of MNC was significant at 40 h of recovery, MMCl induced significant MNC at 12, 20, 24, 40, and 40 h of recovery depending on the concentration. In the second experiment both test chemicals induced MNC which was concentration-dependent in case of MMCl. The highest ineffective concentration tested (HICT) and lowest effective concentration tested (LECT) for MMC determined in this experiment were 0.0005 ppm and 0.001 ppm, respectively. The present work provides evidence that E. crassipes could be a promising in situ environmental biomonitoring assay system.