two related thrombinlike enzymes present in bothrops atrox venom this article describes the presence of two new forms of a thrombinlike enzyme both with apparent molecular masses of 38 kda in bothrops atrox venom both share the ability to cleave fibrinogen into fibrin and to digest casein both present identical km on the substrate bapna their nterminal amino acid sequences are identical for 26 residues sharing 80 homology with batroxobin and flavoxobin two groups of monoclonal antibodies mabs raised against the purified enzyme forms recognized different epitopes of the putative corresponding enzymes present in b atrox crude venom on western blotting analysis of b atrox crude venom mabs 5db2c8 5aa10 and 5cf11 but not mabs 6cc5 and 6ad2g5 revealed two or more protein bands ranging from 25 to 38 kda by immunoprecipitation assays the 6ad2g5 mab was able to precipitate protein bands of 3638 kda from b atrox b leucurus b pradoi b moojeni b jararaca and b neuwiedii crude venoms fibrinogenclotting activity was inhibited when the same venom specimens were preincubated with mab 6ad2g5 except for b jararaca and b neuwiedii