A new crotoxin B isoform PLA(2) (F6a), from Crotalus durissus collilineatus was purified from by one step reverse phase HPLC chromatography using mu-Bondapack C-18 column analytic. The new crotoxin B isoform PLA(2) (F6a), complex crotoxin, the catalytic subunit crotoxin B isoform PLA(2) (F6a) and two crotapotin isoforms (F3 and F4), were isolated from the venom of Crotalus durissus collilineatus. The crotapotins isoforms F3 and F4 had similar chemical properties, the two proteins different in their ability to inhibit of isoforms of PLA(2) (F6 and F6a). The molecular masses estimated by MALDI-TOF mass spectrometry were: crotoxin B: 14,943.14 Da, crotapotin F3: 8,693.24 Da, and crotapotin F4: 9 314.56 Da. The new crotoxin B isoform PLA(2) (F6a) contained 122 amino acid residues and a pI of 8.58. Its amino acid sequence presents high identity with those of other PLA(2)s, particularly in the calcium binding loop and active site helix 3. It also presents similarities in the C-terminal region with other myotoxic PLA(2)s. The new crotoxin B isoform PLA(2) (F6a) contained 122 amino acid residues, with a primary structure of HLLQFNKMIK FETRRNAIPP YAFYGCYCGW GGRGRPKDAT DRCCFVHDCC YGKLAKCNTK WDFYRYSLKS GYITCGKGTW CEEQICECDR VAAECLRRSL STYRYGYMIY PDSRCRGPSE TC. A neuromuscular blocking activity was induced by crotoxin and new crotoxin B isoform PLA(2) (F6a) in the isolated mouse phrenic nerve diaphragm and the biventer cervicis chick nerve-muscle preparation. Whole crotoxin was devoid of cytolytic activity upon myoblasts and myotubes in vitro, whereas new crotoxin B isoform PLA(2) (F6a) was clearly cytotoxic to these cells.