As bites by Bothrops atrox and Lachesis muta muta snakes are frequent in the north of Brazil and elicit similar clinical symptoms, an ELISA assay was developed to identify objectively the circulating antigen in the serum of accidentally bitten patients. Antigens common to the two venoms were removed by immunoaffinity techniques and the 'individual component' of each venom used as immunogen to raise rabbit IgGs. Each of these antibodies specifically recognized one venom, and they were used to set up a sandwich-type ELISA. The specificity of the assay was demonstrated by its capacity to identify correctly the circulating antigen in mice experimentally poisoned with either of the two venoms. The ELISA was further used to identify the circulating antigen in the sera of humans bitten by B. atrox or L. muta muta and to follow the kinetics distribution of antigens in experimentally envenomed mice or accidentally bitten patients. The assay is specific, and could therefore be valuable both to clinicians and to epidemiologists. © 1993.