The haemolysin of Streptococcus suis type 2 Jiangsu isolate was purified. Effects of protein modification reagents on the haemolysin activity were detected. The haemolysin was not affected by PCMB, succinic anhydride (SA), EDC, and N-AI modification, and it indicated that sulfhudryl groups, amino groups, carboxyl groups and tyrosine residues were not essential to haemolysin activity. The haemolysin activity was significantly decreased after NBS, DEPC, 2, 3-Diacetyl, H2O2 modification and was greatly increased after DTT modification. The results indicated that tryptophane residues, histidine residues, arginine residues and disulfides groups seemed to be essential to the haemolysin activity.