In an ultrastructural study of human liver biopsy specimens we found that peroxisomes are regularly present in normal human hepatocytes. Their relationships with the endoplasmic reticulum observed in other species and in other organs were also demonstrable in this material. Some normal peroxisomes were found to display marginal plates or peripheral crystalline inclusions which were present in pathologic specimens as well. In certain inherited metabolic disorders (Menkes' steely hair disease, analbuminemia) the volume of the individual peroxisomes appeared to be considerably reduced. But most pathologic processes affecting hepatocytes seem to produce any or several of the following: increased volume or numbers per cell, changes of shapes, alterations of the consistency of the matrix, appearance of dense numbers per cell, changes of shapes, alterations of the consistency of the matrix, appearance of dense inclusions, or clustering of peroxisomes in some portions of the cytoplasm. Some of these abnormalities are reversible based on observations in three patients with Wilson's disease treated with D-penicillamine. The mean +/- standard deviation of diameters of peroxisomes in four normal subjects was 0.618 +/- 0.143 mum. Significant reductions or increases in mean diameters of peroxisomes were noted in all but two of the 16 pathologic specimens. There were other morphologic abnormalities present in the remaining two specimens. We conclude that various pathologic processes involving the hepatocytic cytoplasm exert different effects on peroxisomes. Although no specific pattern of morphologic alteration emerged from this exploratory study, a possible involvement of peroxisomes ought to be considered whenever metabolic or pathologic processes affect the liver.