The simultaneous adsorption of nickel(II) and lead(II) to Rhizopus arrhizus from binary metal mixtures is described and compared to studies using one metal in solution. The ability of the fungal biomass to bind metals in solution is shown to be a function of pH, the number of metals competing for binding sites in the fungal cells and metal concentrations. The effects of the presence of nickel(II) and lead(II) ions together in aquatic solutions on the biosorption of nickel(II) and lead(II) ions were investigated in terms of initial rates of biosorption, equilibrium adsorption yields and equilibrium isotherms. The effect of lead(II) ions on the uptake of nickel(II) ions was found to be antagonistic, whereas the total interactive effects of nickel(II) and lead(II) ions on the biosorption of lead(II) ions by R. arrhizus can be considered to be synergistic at a defined concentration interval. Lead(II) ions were adsorbed selectively from the binary metal mixtures.