There is a need for increased understanding of metal interactions during chronic metal-mixture exposures so that mixture toxicity effects can be incorporated into regulatory risk assessments. Site waters from the Ranger uranium mine contain a mixture of key contaminants uranium (U), magnesium (Mg), manganese (Mn) and total ammonia nitrogen (TAN). A recent assessment of the toxicity of Ranger mine waters indicated antagonistic interaction occurred in most cases, and Mg and Mn were found to be the two primary contaminants present in free ion form and contributing to toxicity. To investigate this further, the chronic toxicity of binary Mg - Mn mixtures were assessed in full-factorial experiments using three tropical freshwater species (the cladoceran, Moinodaphnia macleayi; green hydra, Hydra viridissima and the green alga, Chlorella sp.) Mixture toxicity was antagonistic at low-effect and high-effect concentrations for both metals, with a progressive increase in the reduction of toxicity as metal concentrations increased. A threshold of amelioration was reached for the hydra, where mixture interaction was still antagonistic, but there was a decrease in the extent to which toxicity of Mg and Mn was reduced. Increased growth and reproduction were observed for the alga and cladoceran exposed to the highest Mg treatments (≥ 3000 mg/L Mg and 63 mg/L Mg, respectively). This amelioration of toxicity that occurs from combined exposure to Mg and Mn, is potentially a result of these divalent cations acting as competitors in their contribution to toxicity. This data confirms that 1) the site-specific guideline values developed for these individual metals will be protective for local aquatic ecosystems downstream of the mine and 2) the antagonism occurring in the mixtures can be attributed to the interaction between the two primary contributors to toxicity: Mg and Mn.