The carboxylic antibiotic lasalocid was shown to facilitate the transport of Cd(II) cations through supported liquid membranes, the driving force being the proton gradient between the two aqueous phases. With hydrocarbons as solvents inside the porous support, the initial metal fluxes were linear versus the inverses of the viscosity values, in accordance with a diffusion limited mechanism. With the polar solvent nitrophenyl octyl ether, the initial flux was six times higher than expected from the viscosity value, and up to 95% of the metal ions were transported. In competitive experiments with equal initial Cd(II) and Zn(II) concentrations, after 94% of Cd(II) had been transported the ratio of Cd(II) to Zn(II) in the receiving phase was 3.8 (pH 8.2 in the source phase, pH 2 in the receiving phase, membrane impregnated with lasalocid 5x10(-3) M in nitrophenyl octyl ether).