Abstract: The effects of reaction time, amount of the NBS, reaction temperature, volume and pH on the labeling efficiency of 125I-Rituximab were systematically studied. The labeling efficiency is more then 92% under the optimum labeling conditions. The labeling efficiency was detected by ITLC-SG. Binding test between Raji cells and 125I-Rituximab were carried out. The results demonstrated that the binding of 131I-Rituximab and Raji cells was time dependent. The biodistribution in mice was detected. High distribution of radioactivity that lasted for 6 days was detected in blood after injection of 131I-Rituximab. It indicate the stability of 131I-Rituximab in vivo. The acute toxicity of the 131I-Rituximab is low.