Discovered and first isolated in 1966 in C. De Duve's§laboratory, peroxisomes - organelles which are present in§nearly all eukaryotic cells - are still not fully§understood.§More than 40 peroxisomal enzymes catalyzing a variety of§reactions have been characterized. Moreover, peroxisomes can§be regarded as toxicological indicators: several compounds,§including hypolipemic drugs, plasticizers or pesticides§trigger their proliferation. This proliferation may lead§to hepatocarcinogenesis in rodents.§Interest in peroxisomes stems not only fromtheir biology,§but also because there is a deficiency of peroxisomal§functions in several genetic diseases. Some genes involved§in inborn errors of peroxisomal function have recently been§identified.§In this manual, based on a FEBS Advanced Course on§peroxisomes, protocols on the following topics are described§in detail: Isolation and characterization of peroxisomes by§ultracentrifugationand immunoblotting; gene regulation§studied by mRNA isolation, hybridizationand DNA cell§transfection; use of cell lines as peroxisome proliferator§targets; transformation with retrovirus; peroxisomes as§toxicological markers; cytochrome P450 induction; drug§design and computer analysis of ligand/receptor interaction§involved in peroxisomal gene expression.