Arathe CONNMON, albeit frequently ignored, finding in hematoxylin-eosin stained sections of human mvocardium is the presence of a basophilic, finely granular material in the cvtoplasm of isolated myocardial fibers. This condition, first described by Ceipel 1 in 1905, has been variously called basophilic degeneration (BD), 2 mucoid degeneration, 3 mucinous degeneration, 4 or cardiac colloid. 5 Despite the fact that a good number of studies have been made on this peculiar condition, 1-14 very little is known of its significance, mechanism of formation and chemical composition. Manion 13 has recently summarized the present, rather unsatisfactory situation regarding the knowledge of BD. In an effort to determine the chemical nature of BD and from there gain insight into the events leading to its formation, we have performed the following histochemical, fine structural and biochemical studies.