Mostly, metal contamination comes as a mixture of different metals. These multi-component mixtures can produce significant mixture effects. In this study, metal mixture toxicity on barley root elongation was investigated. Mixture effects of four metals (Cu, Cd, Ni and Zn) were investigated when each metal was present in the mixture at a low effect concentration, i.e. individually only causing a small effect. Two commonly used ‘reference’ models (concentration addition and independent action) were used to test if it is possible to predict toxicity in these mixtures. In this study, it was shown that the toxicity of a mixture of metals was almost always larger than the effect of the most toxic metal in that mixture. In addition, it was found that a mixture of four metals, each causing individually < 10% effect, produced mixture effects up to 50%. This highlights the need to incorporate mixture toxicity in risk-assessment frameworks.