The Collaborative International Dictionary of English v.0.48:

Antivenin \An`ti*ve"nin\, n. [Written also antivenen,
   antivenine.] [Pref. anti- + L. venenum poison.] (Physiol.
   Chem.)
   The serum of blood rendered antitoxic to a venom by repeated
   injections of small doses of the venom; also, the component
   of such a serum which is antitoxic to the venom.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

antivenin
    n 1: an antitoxin that counteracts the effects of venom from the
         bite of a snake or insect or other animal [syn:
         antivenin, antivenene]