We studied basophilic degeneration of heart muscle cells in 100 consecutive autopsies (74 males, 26 females). Except for two infants, the age of individuals whose tissue was studied ranged from 10 to 90 years (mean age, 68.3). Using histochemical and immunohistochemical methods, we found the frequency and extent of basophilic degeneration and the materials in type IV glycogenosis (Andersen's disease) were reactive to monoclonal antibodies raised against polyglucosan extracted from the myocardium of a patient with Lafora's disease. Basophilic degeneration was found in 98% of the entire study population and in 100% of those aged over 60. The reactivity for polyglucosan became more intense after diastase digestion. Immuno-electronmicroscopy using an immunogold method showed that the fibril-like structures of basophilic degeneration were specifically labelled by the monoclonal antibodies to polyglucosan.