Antibodies against Bothrops asper myotoxin were purified from a polyvalent antivenom by affinity chromatography. These antibodies neutralized most of the myotoxic activity of crude B. asper venom, as judged by histologic evaluation of skeletal muscle and determination of plasma creatine kinase levels in mice. When tested by immunoelectrophoresis, purified antibodies formed two superimposed bands of precipitation against an homogeneous (by SDS-PAGE analysis) preparation of B. asper myotoxin, as well as against crude B. asper venom. Ouchterlony immunodiffusion analysis of purified antibodies showed two precipitation bands with a pattern of complete immunologic identity between samples of crude B. asper venoms from specimens collected in the Atlantic and Pacific regions of Costa Rica. In addition, these antibodies precipitated when reacted against venom of newborn B. asper specimens from the Pacific region, but not against venom of newborn specimens from the Atlantic region. Purified antibodies were tested by immunodiffusion against eleven different snake venoms from Costa Rica. Only the venom of B. schlegelii cross-reacted, indicating the presence in this venom of components immunologically related to B. asper myotoxin. Analysis of purified antibodies to B. asper myotoxin by agarose electrophoresis and by SDS-PAGE suggests the presence of both IgG and IgM on the basis of electrophoretic position and molecular weight of the bands. Results obtained in neutralization experiments suggest that this myotoxin is a major factor in the development of local myonecrosis induced by crude B. asper venom.