In a new case of carnitine palmityl transferase (CPT) deficiency the defect was documented in muscle and muscle cultures with an isotope exchange reaction, using different concentrations of palmityl-DL-carnitine and a forward reaction with and without albumin. The defect was expressed in cultured skin fibroblasts only by the "reverse" and "hydroxamate" reactions. The parents and the patient's daughter had intermediate levels of the enzyme in platelets and fibroblasts, supporting the concept that CPT deficiency has an autosomal recessive pattern of inheritance. The growth pattern and development of muscle cultures in this CPT-deficient patient indicate that CPT activity may be sufficient to allow normal muscle differentiation in culture without lipid storage.