OBJECTIVE To explore the biodistribution and anti-tumor activity of 131 I labeled rituximab injected intratumorally or intraperitoneally in vivo in nude mice bearing Raji human Burkitt ' s lymphoma xenograft s. METHODS The rituximab and the mouse IgG were labeled with Na 131 I using the IODO-GEN method. BALB/C nude mice were xenograft ed with 131 I-Rituximab or 131 I-IgG and killed on the 1st, 3rd, 7th, and 15th day after injection. The tumor/non-tumor ratio (T/ NT) and the dose injected in each gram of the tissue (%ID/g) from 12 organs or tissues of interest, e.g. tumor, blood, were calculated. The long and short axes of each tumor were measured by calipers at 2-3-day intervals aft er treatment, and the growth inhibition of the tumor was calculated using the MIRD formula. RESULTS When comparing intraperitoneal injection (IP) and intratumoral injection (IT) of 131 I-IgG, intratumoral injection of 131 I-rituximab produced a significantly higher tumor/non-tumor ratio in all tissues and organs of interest on the 1st, 3rd, and 7th day, respectively (P < 0.05). The %ID/g of tumor was 1.4-1.7-fold and 1.5-3.7-fold in the IP and IgG IT groups, respectively, but the %ID/g of non-tumors was signifi cantly lower in the IP group and IgG IT group. Similarly, the tumor growth was greatly inhibited by intratumoral injection of the 131 I-rituximab, whereas it was less inhibited by other forms of the treatment (P < 0.05). However 131 I-rituximab injected intratumorally inhibited tumor growth in a dose-dependent manner. The inhibition rate was less with a low dose (75 μCi) and greater with a high dose (150 μCi), yet the diff erence was not signifi cant (P > 0.05). CONCLUSION Tumors can absorb the highest amount of the radiolabelled antibodies, and the tumor/non-tumor ratios in the group with intratumoral injection of the 131 I-rituximab resulted in the optimal anti-tumor activity.KEY WORDS: Iodine-131, anti-CD20 monoclonal antibody, non-Hodgkin's lymphoma (NHL), intratumoral injection, radioimmunotherapy.