Different studies on the toxicity of the same metals to algae have often shown divergent and sometimes contradictory results. The inconsistency of these findings was often attributed to environmental factors such as the degree of chelation, complexation and precipitation of metals. Conventionally, toxicity tests of metal mixtures were conducted by adding metals to algae growing in synthetic media. In our study, we examined the 'in vivo' toxic effects of metals by pretreating the algal cells with one metal, resuspendeding them in fresh medium, then exposing them to a second metal. The algal response showed marked differences between the conventional and the new approach. The conventional approach shows that the toxicity depends upon the complexes formed externally, whereas the in vivo approach shows that the toxicity probably depends upon the molecular transformation of the metals internally.