cytochemical techniques (ICC) which provide a useful adjunct to investigations by immunoblotting. A particular advantage of a cytochemical approach to the investiga­ tion of mitochondrial disorders is that it allows the mosaic distribution of certain of these defects to be detected, whereas the tissue homogeniza­ tion involved in conventional enzyme assays or immunoblotting precludes this. A further advantage of MEA or ICC is that only small amounts of tissue are needed, which is important since many of the affected patients are infants or small children. The main aim of this communica­ tion is to outline ways in which these techniques can be used in the diagnosis and further investigation of mitochondrial disorders. Reference will be made not only to those situations in which MEA and ICC offer advantages over standard enzyme asays and immunoblotting but also to contexts in which the reverse applies. 4. 2 MATERIALS AND METHODS Muscle biopsies for cytochemical investigation were snap-frozen using isopentane cooled to - 150°C in liquid nitrogen. Samples were stored in heat-sealed polythene packets in the vapour phase of liquid nitrogen containers. 4. 2. 1 Microphotometric enzyme assays Frozen sections 8 Jlm thick were cut using a Reichert-J ung Frigocut cryostat microtome equipped with motor-driven cutting action to maintain maximal reproducibility of section thickness. Sections were picked up on microscope slides and air-dried for 15 min at room temperature.