The effects of the basic (CB) and acidic (CA) subunits of crotoxin on rat skeletal muscle in vivo have been studied. CB caused muscle necrosis, with damage limited to an outer rim of muscle fibres in rat soleus. The inflammatory response was relatively small. CA was apparently inactive, but the effect of CB was potentiated if injected in combination with CA. Not only were more muscle fibres damaged, but the damage was more diffuse. There was no relationship between the number of muscle fibres damaged and the extent of the oedema (measured as increase in wet weight.