Complex I deficiency is probably the most common enzyme defect among the group of OXPHOS disorders. To evaluate a deficiency of complex I activity, biochemical measurements based on estimation of the mitochondrial rotenone-sensitive NADH: ubiquinone oxidoreductase activity are an important tool. Skeletal muscle is the most widely used tissue to examine complex I deficiency. However, obtaining a muscle biopsy requires an invasive surgical operation. It is much easier to obtain blood lymphocytes or skin fibroblasts, and, moreover, these cells can be expanded in number by standard techniques for extensive research on complex I. On the other hand, each of these cell types has disadvantages that hinder its measurement, such as the apparent low enzyme activity of lymphocytes and the highly contaminating nonmitochondrial NADH-quinone oxidoreductase activity of fibroblasts. This chapter describes a method to assay complex I activity reliably in a minute amount of either cell type.