β-Agkistrodotoxin (β₁-AgTx), a presynaptic neurotoxin obtained from the venom of snake Agkistrodon blomhoffii brevicaudus (Jiangshu and Zhejiang Province, China) was further purified by the method of preparative electrophoresis. The polymerism was found in the native component of β₁-AgTx and the purified β₁-AgTx-OH^- which was solubilized in the slight basic solution (pH 7.5) by SDS-PAGE analysis, however, the amount of polymers seems to be stable observed in the different days. Moreover, by two-dimensional electrophoresis analysis, it was found that both components of β₁-AgTx and β₁-AgTx-OH^- were composed of three isoforms (pI 7.0, 5.8 and 5.4) with different contents, and the rate of content of isoforms in the component of β₁-AgTx-OH^- was changeable observed in the different days, but not in the β₁-AgTx.