The activity of carnitine palmitoyl transferase, an enzyme that catalyzes the transport of long-chain acylcarnitines into mitochondria, was quantitated in EB-virus-transformed lymphoblasts from 7 patients with susceptibility for malignant hyperthermia. Immunoreactive enzyme protein was also measured using an enzyme-linked immunosorbent assay. Cell lines derived from patients with carnitine palmitoyl transferase deficiency of muscle and from normal individuals were used as positive and negative controls, respectively. One patient with malignant hyperthermia had a deficiency in the enzyme activity which was comparable with that of the known carnitine palmitoyl transferase deficient patients. This individual's lymphoblasts were also deficient in immunoreactive enzyme protein. All of the remaining patients with malignant hyperthermia were deficient only when the backward assay for carnitine palmitoyl transferase was used for quantitation. It is likely that a subset of individuals with a malignant hyperthermia phenotype have a primary deficiency of carnitine palmitoyl transferase and that others have a milder enzyme deficiency secondary to the primary defect in malignant hyperthermia.