Ionizing radiation is able to detoxify several venoms, including snake venom, without affecting significantly their antigenic and immunogenic properties. In order to elucidate this phenomena, we conceived a comparative biochemical and pharmacological study between native and gamma irradiated (2000Gy) crotoxin, main toxin of south american rattlesnake Crotalus durissus terrificus. Crotoxin was isolated and purified from crude venom by molecular exclusion chromatography, pI precipitation and then irradiated. Immunodiffusion, electrophoresis and gel filtration showed that the molecular integrity was preserved after irradiation with some higher molecular weight aggregate formation and maintenance of its antigenic capacity. The antibodies induced by irradiated toxin had a similar titer to the antibodies induced by native crotoxin; however with higher protective effects in mice. Crotoxin toxicity became 15 times lower after irradiation, as determined by LD sub(50) in mice. Native and irradiated crotoxin biodistribution occurred with a similar general pattern, with renal elimination. In contrast to irradiated crotoxin, the native crotoxin is initially retained in kidneys. A later concentration (2-3hs) occurs in phagocyticmononuclear cells rich organs (liver and spleen) and neural junctions rich organs (muscle and brain). (author).