The cation selectivity profiles of the carboxylic ionophores, carriomycin, lonomycin and etheromycin, have been investigated by measuring the complexation affinities for metal cations and the cation transport activity through an organic barrier. In a two-phase partition study, carriomycin and lonomycin formed complexes more readily with K⁺ than with NH₄⁺, Rb⁺ or Na⁺, but not with Li⁺ or Cs⁺. On the other hand, etheromycin exhibited a great preference for K⁺ or NH₄⁺ over Na⁺, Li⁺ or Rb⁺, but displayed no binding affinity for Cs⁺. The alkaline degradation product of lonomycin exhibited a preference for K⁺ or Na⁺, but its complexation affinities were much lower than those of the parent compound. Carriomycin, lonomycin and etheromycin efficiently transported K⁺, Rb⁺ and Na+ through a CC1₄ barrier. But did not carry Ca²⁺. These antibiotics caused a massive release of K⁺, Rb⁺ or Na⁺, but not of Li⁺ and Cs⁺, from mitochondria previously loaded with these cations by valinomycin or monazomycin. Thus, it is concluded that carriomycin, lonomycin and etheromycin are monovalent cation selective ionophores.