The biological activity of antiserum against venoms can be assessed by "in vivo" tests using pigeons, rabbits, guinea-pigs and adult mice although there is no International standardized reaction for it. The traditional methods using pigeons and adult mice were comparatively studied with those using mice of 4-5 g (6-7 days of age), inoculated by subcutaneous injection. The toxicity of the Crotalus durissus terrificus venom was initially determined by studying the symptomatology after administration of venom and lethal activity. The comparative study of these three methods showed that the best concordance in the results of the ED100 was obtained using adult and baby mice. The most homogeneous ED100 and LD50 results were found in baby mice when compared with those found in a in adult mice and pigeons. With this new method we can avoid the difficulty in obtaining those animals, besides utilization of a great number of animals per biological evaluation point reaching better precision and a great unformity, required in this type of dosage like weight, age and strains, in order to get better reproductibility of results.