R. Batzri-Izraeli and A. Bdolah. Isolation and characterization of the main toxic fraction from the venom of the false horned viper (Pseudocerastes fieldi). Toxicon 20, 867-875, 1982-The venom of Pseudocerastes fieldi was subjected to gel filtration on Sephadex G-75. Most of the protein and lethality of the venom were eluted in a major symmetrical peak (C). The lethality of this peak is confined to a basic protein fraction, Cb (pI > 9.5) separable by DEAE-cellulose chromatography. Two proteins with molecular sizes close to 16,000 daltons were isolated from this fraction by preparative acidic gel electrophoresis in the presence of Triton X-100. One of the proteins (CbII) is lethal to mice (ld50 = 1 mg/kg) and shows phospholipase A activity as well as direct hemolytic activity. The other protein (CbI) does not reveal any known biological activity. However, upon recombination of the two a synergistic lethal activity is evident (the ld50 of the mixture = 0.25 mg/kg). It is suggested that CbI may be a specifier which potentiates the toxicity of the phospholipase A at the target site.