Abstract: Monoclonal antibodies(McAbs)were generated by fusion of SP2/0 myeloma cells with spleen cells from BALB/c mice immunized with Giardia lamblia trophozoites.To define the effect of the McAb on Giardia lamblia ,trophozoites were subcultured axenically after contact with McAb for 1 h at 37℃.The McAbs 1G 5 and 1E 2(both IgG 1)were able to inhibit the growth of trophozoite at the rate of 63.4% and 35%, respectively.The inhibitory effect was decreased along with the decreasing quantity of McAb(from dilution 1∶45 to 1∶225)added to the TYI S 33 medium.The actions of McAbs could be eliminated by absorption with trophzoites and not be influenced by complement in fresh guinea pig serum,thus the actions of McAbs appeared to be specific and complement independent.