Synergistic and antagonistic effects on genotoxicity of mixtures of four chemicals; i.e., lead tetraacetate (LTA), arsenic trioxide (ATO), dieldrin (DED), and tetrachloroethylene (TCE), were evaluated by the Tradescantia-micronucleus (Trad-MCN) assay. The chemicals were mixed in ratios of 1:1, 1:2 and 2:1 for mixtures of two chemicals and 1:1:1 each for three chemicals. The concentration of stock solution of these chemicals was around the minimum effective dose (MED) or below the MED for these chemicals as reported by Sandhu et al. (1989). Treatments were applied to plant cuttings by hydroponic uptake of the mixed solutions through the stems of the plant for 30 h followed by fixation of the flower buds in aceto-alcohol (1:3 ratio) without a recovery period. Microslides were prepared for scoring MCN frequencies. Results of two series of repeated experiments indicated that all mixtures of LTA/ATO exhibited antagonistic effects. On the other hand, all mixtures of TCE and DED exhibited synergistic effect. These data indicate that for evaluating biological hazards at chemical waste sites, it is prudent to evaluate the genotoxicity of complex chemical mixtures as these exist in nature because the biological effects based on evaluating individual chemicals may not be true predictors of the interactive effects of the pollutants. (Copyright (c) 1992 Elsevier Science Publishers B. V.)