The formation of new microbodies induced by partial hepatectomy was studied in the liver of male rats using conventional microscopy and the peroxidase reaction for demonstrating catalase. In the early phases of the regenerative period, small smooth walled vesicles were identified adjacent to mature microbodies and these contained material similar both in appearance and in endogenous peroxidase activity to the neighboring microbody. Membrane continuity between the two organelles was identified clearly. It is suggested that the formation of new microbodies from pre-existing microbodies may be an important general method of microbody proliferation.