A monoclonal antibody (H₆₅) was coupled to ricin using three different cross-agents respectively. Immunotoxins (ITs) with greater cytotoxic potency were obtained through a proper scheme. The ITs reduced the ³H-leucine incorporation of Molt-4 cells by 50％ at a concentration (IC₅₀) of 2.7×10^-12mol/L. ITs prepared with disulfide bond is 5—10 fold more cytotoxic in vitro than those prepared with thioether bond. The speed of metabolism of the latter in vivo is two times slower than that of the former. Because these two kinds of ITs have different degradation products, their metabolism pathways in vivo may be different also.