On the basis of the interaction between tritiated PGBx and rat liver mitochondria, it appears that PGBx interacts with rat liver mitochondria to form a complex. At low PGBx-mitochondrial ratios, one effect of this complex formation is to stabilize the phosphorylation activity of rat liver mitochondria when exposed for short times to hypotonic solutions. At higher PGBx-mitochondrial ratios, PGBx fails to show this effect. At low PGBx concentrations, PGBx is also shown to inhibit release of amino acids and proteins as well as glutamic acid dehydrogenase and monoamine oxidase from the mitochondria and to inhibit mitochondrial swelling.