Following a single i.m. injection of a sublethal dose of crotoxin into the hindlimb of mice, degenerative changes were observed in the soleus muscle in the ensuing 72 hr using light and electron microscopy. A similar degree of myonecrosis resulted from the injection of the phospholipase A2 subunit alone, whereas crotoxin reconstituted from a chemically modified phospholipase A2 subunit with low catalytic activity showed weak myonecrosis and the non-enzymic subunit, crotapotin, was inactive. Visible defects of the sarcolemma were associated with underlying necrotic areas and attraction of macrophages. Internally, hypercontraction bands were often present and mitochondria in the rounded, orthodox configurational state contained linear dark bodies as part of their crystal organization. In fibre segments with slow onset of degeneration, many mitochondria additionally contained clusters of electron opaque deposit and in mitochondria, heavily laden with such deposits, energy-dispersive microprobe analysis showed the presence of high concentration of calcium. It was concluded that the primary site of action of crotoxin was hydrolysis of the sarcolemmal membrane and that mitochondrial changes were associated with calcium accumulation in response to an altered internal environment. Regeneration of fibres was rapid.