In addition to being found in peroxisomal diseases, peroxisomal alterations are also seen in viral hepatitis, though quantitative data are lacking. Experiments were performed on BALB/c mice. These mice were infected with Mouse Hepatitis Virus type 3 or were starved. The peroxisomes were cytochemically stained for catalase. Light microscopic, ultrastructural and morphometric analysis were performed. Several peroxisomal changes were observed 24 h after infection, and these changes became more pronounced after 40 h. There was a decrease in catalase activity, which was more pronounced in some regions, in some cells and in individual organelles; and there was also the onset of a progressive decrease in the number of organelles. It is believed that peroxisomes disappear by lysis. Proliferation probably occurs simultaneously up to 40 h after infection. At 48 h, necrotic foci are found to have swollen peroxisomes, and thus destruction is enhanced. Although peroxisomes seem to be sensitive markers of hepatic injury, they show a heterogeneous reaction pattern. Our results are discussed in relation to human viral hepatitis. © 1990.