Snake venoms are a complex mixture of components, which have a wide range of actions both on prey and human victims. The genus Bothrops causes the vast majority of snakebites in Central and South America, being responsible for 80% of snake envenomations in Brazil. Envenomations are characterized by prominent local effects, including oedema, haemorrhage and necrosis, which can lead to permanent disability. Systemic manifestations such as haemorrhage, coagulopathy, shock and acute renal failure may also occur. In the present study we have investigated the action of venoms from 19 species of snakes from the genus Bothrops, occurring in Brazil, on the complement system in in vitro studies. All venoms were able to activate the classical complement pathway, in the absence of sensitizing antibody. This activation was in part associated with the cleavage of C1-Inhibitor by proteases present in these venoms, which disrupts complement activation control. No modification of the membrane bound complement regulators, such as DAF, CR1 and CD59 was detected, after treatment of human erythrocytes with the snake venoms. Some of the Bothrops venoms were also able to activate alternative and lectin pathways, as measured in haemolytic and ELISA assays. C3a, C4a and C5a were generated in sera treated with the venoms, not only through C-activation, but also by the direct cleavage of complement components, as determined using purified C3 and C4. Metallo- and/or serine-protease inhibitors prevented cleavage of C3 and C4. These results suggest that Bothrops venoms can activate the complement system, generating a large amount of anaphylatoxins, which may play an important role in the inflammatory process presented in humans after snake envenomations, and they may also assist, due to their vasodilatory effects, to enhance the spreading of other venom components.