Abstract: Objective To study the biological response of B-cell lymphoma cells positive for CD20 expression to 131I-labeled rituximab. Methods Anti-CD20 monoclonal antibody rituximab was labeled with 131I by means of IODO-GEN method, and its effects on apoptosis of Raji cells were determined by Annexin-V/PI double-labeled cytometry. Its effects on the cell cycles was evaluated by cytometry with PI staining. Results The cell apoptosis rate measured by Annexin V-FITC/PI was 51.99% in 131I-rituximab group, significantly higher than that in 131I group, rituximab group and control group (42.71%, 29.42% and 26.17%, respectively, P<0.05). The apoptosis rate by flow cytometry with PI staining was 4.32% in 131I-rituximab group, also significantly higher than that in the other 3 groups (1.47%, 1.39% and 0.37%, respectively, P<0.05). Cell cycle alteration of Raji cells occurred in 131I-rituximab group, and the majority of cells were arrested at G1/G2 stage. Conclusion 131I-rituximab can regulate the cell cycle of Raji cells and induce their apoptosis to inhibit their proliferation.