The extent to which Ca(2+), Mg(2+), Na(+), K(+) ions and pH independently influence copper toxicity to barley (Hordeum vulgare) was assessed by measuring root growth in nutrient solutions. Increased Ca(2+) activity resulted in a sixfold decrease in [EC50(cu2+)] values, while a positive relationship between the cation activity and the EC50 was expected. Increased Mg(2+) activity resulted in a twofold increase in [EC50(cu2+)] values. Na(+), K(+) and H(+) activities did not significantly affect Cu(2+) toxicity. The obtained results indicated that competition for binding sites between Cu(2+) and cations such as Ca(2+), Mg(2+), Na(+), K(+) and H(+) is not an important factor in determining Cu(2+) toxicity to H. vulgare. However, the EC50s could, with one exception, be predicted within a factor three based on the free Cu(2+) activity, indicating that the free Cu(2+) activity cannot only be used to predict metal toxicity to aquatic, but also to terrestrial organisms.