The toxicity of two toxic elements, arsenic (As) and cadmium (Cd) (individually or in combination) on root elongation of wheat seedlings (Triticium aestivum, L.) were investigated both in hydroponics and in soils freshly spiked with the toxic elements. Median effective concentration (EC50) and non-observed effect concentration (NOEC) were used to investigate the toxic thresholds and potencies of the two elements. The EC50 for As was 0.97 mu M in hydroponics and 196 mg kg(-1) in soil, and 4.32 mu M and 449 mg kg(-1) for Cd, respectively. Toxic unit (TU) and additive index (Al) concepts were introduced to determine the combined outcomes, and different behaviors were obtained: synergism in solution culture (EC50mix = 0.36TU(mix) and AI: 1.76) and antagonism in soil experiments (EC50mix = 1.49TU(mix) and Al: -0.33). Furthermore, the data of soil bioavailable As and Cd cannot explain the discrepancy between the results derived from soil and hydroponics experiments. (C) 2007 Elsevier B.V. All rights reserved.