A 13-year-old boy developed cramps and myoglobinuria following exertion. Mitochondrial preparations from a skeletal muscle biopsy were deficient in carnitine palmitoyltransferase (CPT) activity when assayed by the hydroxamate and kinetic assays. The patient's fibroblasts were also deficient when assayed by the hydroxamate and kinetic assays, but not when tested by the DTNB (5,5'-dithiobis-[nitrobenzoic acid]) method. This disparity probably indicates a specific deficiency in fibroblasts of one of the two carnitine palmitoyltransferases, presumably CPT II.