This paper presents a collection of quantitative values for respiratory chain activities in human tissues. These were measured in the most widely used tissues in screening procedures for respiratory chain deficiencies. Investigations were mainly carried out using the different standardized micro-methods previously detailed (Rustin et al., Clin Chim Acta, 1994). The potential effect of the age of the patients on both absolute and relative levels of respiratory chain activities in their skeletal muscle tissue was first considered. No evidence for any significant difference between the various age groups in the studied population (ranging from 0 to above 50 years of age) was observed. Moreover, a quite similar picture of the organization of the respiratory chain was suggested independent of the tissue or the cells investigated. In particular, it was found that roughly identical enzyme activity ratios could be measured in all tissues, which allowed study of the differential involvement of organs and tissues in patients potentially affected by a respiratory chain deficiency. Some tissue-specific features were, however, observed, including varying rates of glycerol-3-phosphate dehydrogenase activities and increased succinate dehydrogenase activity in liver. The technical limitations remaining in the investigations of respiratory chain disorders in man are discussed in the conclusion.