A study of mitochondrial oxidative phosphorylation in biopsies from human hepatocellular carcinoma is presented. Tumour mitochondria as compared to control liver mitochondria, besides a reduced activity of complex IV (cytochrome c oxidase) of the respiratory chain, show a decreased phosphorylative capacity. This appears to be mainly related to a defective F o F(1) ATP synthase complex. Use of an antibody against the F(1) portion of the complex demonstrate a definite decrease for the beta subunit of F(1) in tumour mitochondria.