The correlation coefficients between in vivo neutralization of lethal toxicity (ED50), neutralization of the hemolytic activity (PLA2) and levels of antibodies measured by ELISA, was investigated to test the potency of horse anti-bothropic antivenom. Twenty six horses were hyperimmunized with Bothrops venoms (B. alternatus, B. jararaca, B. jararacussu, B. neuwiedii and B. moojeni). To set up an indirect ELISA, for neutralization of PLA2 activity and for determination of ED50 in Swiss mice, the whole Bothrops jararaca venom (reference venom for assessing the bothropic antivenom potency in Brazil) was used. The toxic fraction (purified from B. jararaca venom by Sephadex G-100 chromatography) was also used as antigen for ELISA. All antivenoms analyzed effectively neutralized the lethal activity in the range of 1.6 to 9.6 mg/ml of antivenom. The correlation coefficient between ED50 and ELISA antibody titers against the crude venom and toxic fraction was r = 0.65 (P < 0.001) and r = 0.85 (P < 0.0001), respectively. Correlation between ED50 and neutralization of PLA2 activity was r = 0.52 (P < 0.01), and the correlation between ELISA antibody titers and neutralization of PLA2 activity was r = 0.58 (P < 0.002). Thus, the ELISA which measures only the antibody against the major toxic fraction of the B. jararaca venom should be most suitable for use as an in vitro assay of bothropic antivenom potency.