A 33-year-old female complained of muscle pain and stiffness after severe exercise from the age of nine. Her sister also had similar symptoms. Consanguinity was found in her parents. Neither muscle wasting nor weakness was detectable. The activity of LDH in the muscle was decreased less to than 8% of normal value. The isoenzyme pattern of the muscle LDH revealed only one band of B4. The levels of blood lactate did not rise on anaerobic exercise, while a marked increase of pyruvate was found. Northern blot analysis showed that the ratio of LDH-A transcript to beta-actin transcript in the patient was similar to that in a normal subject. RNA preparations were reverse-transcribed, amplified by a polymerase chain reaction (PCR), and separated by electrophoresis. The size of PCR product corresponding to exon 6 was decreased. The nucleotide sequence of this product was determined and a 20 bp deletion was found. This mutation results in a frame-shift translation and premature termination. The predicted incomplete LDH-A subunit contains only 259 instead of 331 amino acids. Immunofluorescence for LDH-A subunit could be seen within the cytoplasm and on the surface membrane of the muscle fibers in our patient as well as control subjects by the immunohistochemical studies. These findings suggest that LDH-A mRNA is transcribed in a truncated form and an enzymatically inactive protein is produced in the patient's muscles.