The resistance of several animals to snake venom has been reviewed. Some general concepts are introduced to allow the comparative evaluation of the resistance of different animals studied by different investigators. The purification and properties of several factors isolated from the serum of different animals by some researchers are described: Trimeresurus flavoviridis (Omori-Satoh et al., 1972); Vipera palaestinae (Ovadia et al., 1975, 1977); Sigmodon hispidus (Pichyangkul and Perez, 1981); Didelphis virginiana and Didelphis marsupialis (Menchaga and Perez, 1981; Moussatché et al., 1979, 1980, 1981; Perales et al., 1986, 1989a,b); Neotoma micropus (Garcia and Perez, 1984); Erinaceus europaeus (de Witt and Weströmm, 1987); Herpestes edwardsii (Tomihara et al., 1987); Dinodon semicarinatus (Tomihara et al., 1988); and Philander opossum (Domont et al., 1989). The protective antihemorrhagic and antineurotoxic factors have some common characteristics: they are acid proteins with isoelectric points ranging between 4.0 and 5.4; their molecular masses vary from 52 to 90 kDa, with one exception of 780 kDa; none has proteolytic activity; their pH and thermostabilities are high and they seem to be glycoproteins. No precipitation lines are formed between the neutralizing proteins and the venoms upon immunodiffusion, indicating that the serum protective factors are not immunoglobulins. The possible mode of action of the antineurotoxic factor isolated from Vipera palaestinae by Ovadia et al. (1977) is shortly discussed as well as the possibility that the antihemorrhagic factors may act by a similar mechanism.