[en] The authors have previously demonstrated the inhibitory effect of rhodamine 123 (Rh123) on mitochondrial bioenergetic function and established mitochondrial F₁-F₀ ATPase as the primary target site for Rh123 toxicity. Further, they found that the amount of Rh123 taken up by isolated rat liver mitochondria is a linear function of mitochondrial membrane potential. Here they investigated the possibility that the differential cytotoxicity exhibited by Rh123 for certain carcinoma cell types might be a function of an increased mitochondrial membrane potential. Membrane potential in mitochondria isolated from cultured cells was measured by equilibrium distribution of ⁸⁶Rb. Mitochondria from CX-1, a Rh123-sensitive carcinoma cell type, had a higher membrane potential (163 +/- 7mV) than did mitochondria from CV-1, a Rh123-insensitive, normal epithelial cell type (104 +/- 9mV). In addition polarographic determination of respiratory activity of mitochondria isolated from these cells showed that CX-1 mitochondria were more sensitive than CV-1 mitochondria to inhibition by Rh123. Finally, there was no evidence of differential sensitivity of F₁-F₀ ATPase activity to Rh123 in CX-1 vs. CV-1 mitochondria. These data strongly suggest that inherent differences in mitochondrial membrane potential contribute to the differential cytotoxicity exhibited by Rh123 for certain carcinoma cell types