The toxicity of CuSO₄·5H₂O, K₂Cr₂O₇, ZnSO₄·7H₂O, and Ni(NO₃)₂ and their binary mixtures on L. sativum and S. polyrrhiza was determined. The type of toxic interaction at each tested metal combination was evaluated by a statistical approach on testing the null hypothesis of “additive toxicity” at p<0.05. The effects were defined as antagonistic, additive, or synergistic, in accordance with the sign of difference between the tested hypothesis and the value of the observed toxicity at tested combinations. In the majority of metal combinations, the interactions for L. sativum were of antagonistic nature (94%), and only 6% of additive. The 100% additive interactive effects were found in the metal mixtures for S. polyrrhiza. The antagonistic or additive interactive effects found in almost all metal ion mixture combinations confirms the presumption that the interaction between ions can be caused by competition for the same reaction center on cell membranes if these ions belong to the same group of Lewis acids.