Abstract: A method of diagnosing cystic fibrosis, identifying carriers for cystic fibrosis, and non-carriers or "normal" persons. Heretofore, no carrier or prenatal detection procedure for cystic fibrosis existed. The detection method is based on the discovery of the genetic abnormality or biochemical defect in cystic fibrosis, which occurs in mitochondria, minute bodies found in the cytoplasma of most cells which are the principal energy source of the cell and contain the cytochrome enzymes of terminal electron transport. The method comprises an assay carried out on preparations derived from human cells possessing mitochondria. The assay may be a kinetic assay of the enzyme complex of the energy conserving site of the mitochondrial electron transport system of the cells or an assay of mitochondrial activity governed by the enzyme complex. The assay is then evaluated by comparison with standards established as the result of similar assays of cells of other subjects of known condition.