Phosphodiesterase (PDE) enzyme activity was screened in four Egyptian snake venoms belonging to families Elapidae and Viperadae and a highly active enzyme (Nn-PDEII), from the richest snake venom species Naja nigricollis, was purified and characterized in a set of biochemical, immunological and biological assays. Purification was followed in two successive steps including gel filtration and ion exchange chromatography. The enzyme exists as a dimer with a disulphide bridge between the two subunits. The enzyme showed its optimum activity at 60 oC and pH 8 and was best stable in alkaline medium (pH 8-9) at 40 oC up to 1 h, however, it loses ~50% of its activity at 60 oC. PDE activity was markedly increased by Mn+2, Co+2 and Mg2+ , increased to a lesser extent by Ca2+, Ba2+, Ni2+, K+ and Na+ but not significantly affected by Zn2+ ions and iodoacetic acid. On the other hand, Al3+ and Cu2+ ions, PMSF, DTT, EDTA, O-phenanthroline and L-cysteine exhibited obvious inhibitory effects. Nn- PDEII was highly immunogenic to rabbits and could induce antibodies specific to N. nigricollis venom antigens. In addition, anti-PDEII antibodies were able to partially neutralize, in vitro, PDE activity of the enzyme in a dosedependent manner. Nn-PDEII exhibited potent anticoagulant effect most probably via inhibiting one or more of the coagulation factors within the intrinsic pathway in the coagulation cascade. Characterization of a component of N. nigricollis venom that affect blood coagulation at specific stage could have potential in identifying new drug leads in the treatment of hemostatic disorders.