We studied the antibodies hepatocyte paraffin 1 (Hep Par 1) and thyroid transcription factor-1 (TTF-1; clone 8G7G3/1) in normal human liver tissue with immunoelectron microscopy using renal tubules as control samples. TTF-1 (8G7G3/1) and Hep Par 1 bound exclusively to the mitochondria of hepatocytes but not to those of the renal tubular epithelium. Both antibodies labeled mitochondria in a similar pattern and density. These findings confirm that the binding site of Hep Par 1 in hepatocytes is mitochondrial. The specific binding of TTF-1 (8G7G3/1) in hepatocyte mitochondria suggests its potential usefulness for identifying hepatocellular carcinoma. Western blot analysis with cellular proteins extracted from normal human liver and thyroid tissue demonstrated that Hep Par 1 and TTF-1 (8G7G3/1) bound to a protein band of approximately 150 kd in liver cells, with TTF-1 (8G7G3/1) showing less affinity. It is likely that different epitopes to Hep Par 1 and TTF-1 (8G7G3/1) share the same protein molecule in hepatocyte mitochondria.