Limb girdle muscular dystrophy 2B and Miyoshi myopathy were recently found to be allelic disorders arising from defects in the dysferlin gene. We have developed a new diagnostic assay for limb girdle muscular dystrophy 2B and Miyoshi myopathy, which screens for dysferlin expression in blood using a commercially available monoclonal antibody. Unlike current methods that require muscle biopsy for immunodiagnosis, the new method is simple and entails a significantly less invasive procedure for tissue sampling. Moreover, it overcomes some of the problems associated with the handling and storage of muscle specimens. In our analysis of 12 patients with limb girdle muscular dystrophy 2B or Miyoshi myopathy, the findings obtained using the new assay are fully consistent with the results from muscle immunodiagnosis.