Individual differences in venom composition among several Tityus serrulatus specimens collected in the same area were examined by enzyme-linked immunosorbent assay (ELISA). Polyclonal antibodies raised against whole venom and against the α-type toxin (toxin IV-5 or Ts IV) and the β-type toxin (toxin γ or Ts VII) were used to study specific variations in the venom. The ELISA results indicated clear differences among the scorpion venoms examined. The lethality (LD50) determined by subcutaneous injections of pooled venom with the same characteristics showed an interesting correlation between the expression level of each component studied and the lethal effect of the venom. Among the groups analysed, the group with the highest concentration of a-type toxin showed the highest toxicity. The groups with the lowest level of toxicity were those with a low concentration of a-type toxin. The results show that the lethality of the venom varies from specimen to specimen and suggest that α-type toxin must be the major lethal component in the whole venom.