Polyborate anions were found to inhibit mitochondrial ATPase. Mercapto and chloro derivatives of dicarbononaborates showed full inhibition of the enzyme activity at 0.5-0.8 mM. The inhibitory effect of dodecaborates was lower. The inhibition was of competitive type with respect to ATP. The inhibition of soluble F1-ATPase indicates a direct interaction of the polyborate anion with the catalytic part of the enzyme molecule.