This paper describes our present strategy for the investigation of respiratory chain disorders in humans. Because very few of the underlying mutations causing mitochondrial disorders in humans are currently known, biochemical studies constitute a major tool in screening procedures for respiratory chain deficiencies. All biochemical and molecular methods described are scaled-down methods, allowing investigation in both adults and young children. Polarographic studies and/or spectrophotometric studies on whole cells (circulating lymphocytes), isolated mitochondria (skeletal muscle) and tissue homogenates are presented. Advantages and limitations of each approach, as well as useful parameters for the characterization of defects and comparison between various tissues are discussed.