The ability of horse antivenoms, consisting of immunoglobulin G (IgG) and its fragments F(ab') 2 and Fab were comparatively studied in mice to neutralize several effects of Cerastes cerastes venom. The three antivenoms were produced from the same batch of hyperimmune horse plasma. Neutralization was only partial when antivenins were administered intravenously at various time intervals after envenomation. No significant differences were observed among IgG, Fab, and F(ab') 2 antivenoms concerning neutralization of hemorrhagic effects. Fab fragments were slightly more effective in neutralizing edema while IgG and F(ab') 2 antivenoms were better in neutralizing myonecrosis in experiments involving independent injection of venom and antivenom.Thus these results disagree with the theory that "Fab " fragments are more effective than whole IgG and F(ab') 2 in the neutralization of local symptoms accompanying C. Cerastes venom.