Abstract: The study is to observe the effective half life of 131I -rituximab in different patients and to establish a individual radioimunotherapy dosage. The biologic distribution and clearances of 131I -rituximab in 13 CD20 positive B cell NHL(non-Hodgkin's lymphoma, NHL) patients are detected by plane scans, and the whole-body counts are collected for drawing time-radioactivity curve, based on which the whole body clearance rate(A) and the whole-body effective half life(Teff)of each patientareare obtained. And then whole-body radiation dose in imaging dose level could be calculated by MIRD (Medical Internal Radiation Dosimetry, MIRD)formula. Finally, by making 75 cGy at the maximally tolerated whole-body radiation dose for a 70-kilogram-weight man as a reference, the maximally individual administered radioactivity could be achieved. For all of the thirteen patients, the range of Teff is 58. 73-177. 69 h(96. 87±30. 58 h), whose distribution seems to be normal, and Teff of most patients is 90-120 h. The range of maximally individual administered radioactivity is 1. 14-2. 95 GBq, whose distribution is irregular. One of these patients is treated with this method and demonstrates a partial response.