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.