The two major types of lesions in myofibrillar myopathy consist of hyaline spheroidal structures composed of compacted myofibrillar residues, and nonhyaline lesions that comprise foci of myofibrillar destruction. We employed immunocytochemical analysis to further characterize these abnormalities. The nonhyaline lesions are depleted of actin, alpha-actinin, myosin, and, less consistently, of titin and nebulin. Thus, each major component of the myofibrils is lost or decreased. These lesions also react strongly for both NCAM and desmin. By contrast, the hyaline structures are highly enriched in actin, are immunoreactive for fast and slow myosin, and show increased expression of titin, nebulin, and alpha-actinin. They fail to react for NCAM and react variably for desmin. Both types of lesion react, but with differing intensities, for gelsolin, dystrophin, beta-amyloid precursor protein (beta APP) epitopes amino-terminal to the alpha-secretase site, alpha 1-antichymotrypsin, and ubiquitin, and both can be congophilic. The increased expressions of desmin, dystrophin and gelsolin in muscle are also confirmed by immunoblot studies. The results, in harmony with the ultrastructural findings described in the companion paper, suggest that myofibrillar myopathy is conditioned by abnormal activation of a degradative process that primarily affects the myofibrils. A structural abnormality of desmin alone may not be sufficient to disrupt the myofibrillar architecture, but abnormal activation of a phosphorylating process could account for dissolution of the myofibrils. The cause and significance of the ectopic overexpression of desmin, dystrophin, NCAM, and beta APP components, and the chemical basis of the congophilia remain unknown.