By virtue of the lanthanum nitrate staining technique applied to biopsied muscle we are able to demonstrate interaction between satellite cells and parent myofibers, as well as development of premyocytes from activated satellite cells. The process of regeneration in diseased muscle appears to differ from that described in experimental myogenesis. Transformation of activated satellite cells to two types of premyocytes in the process of muscle regeneration seems to rely primarily on the state of innervation and recovery rate of the parent cell after injury. Activated satellite cells are characterized morphologically by proliferation of caveolae, first on the parent fiber side, and early T-tubule and myofilament formation and central displacement. In diseased human muscle the satellite cells appear to play significant roles in muscle regeneration both as a source of reinforcement for failing metabolism in the parent cell and as potential replacements for the necrotic segment of the parent cell. This study also demonstrates that the satellite cells are capable of developing into independent myocytes which may fuse with or replace the parent cell, dependent upon the type and extent of the injury sustained. Abnormal fusion among premyocytes or with their parent fiber, resulting in formation of split-or ring-fibers, becomes conceivable when both innervation and recovery from the injury of the parent cell are delayed. Thus, myotube formation, characteristic of usual myogenesis, seldom takes place in the regenerative process instituted by satellite cells in diseased human skeletal muscles.