Background: This work aimed to evaluate the influence of two chelators: DOTA(SCN) and DOTA(NHS) on radioimmunotherapy using 177 Lu-DOTA-Rituximab preparations in murine lymphoma xenograft models. Subsequently, based on animal data, the organ radiation-absorbed doses were extrapolated to humans (adult male). Materials and Methods: Therapeutic efficacy of 177 Lu-DOTA-Rituximab was evaluated in male nude mice bearing either Raji (B lymphocyte, CD20 +) and Jurkat (T lymphocyte, CD20) xenografts, utilizing an anti-CD20 antibody-Rituximab conjugate with either DOTA(SCN) or DOTA(NHS). The DOTA-Rituximab conjugates were prepared in the form of freeze-dried kits. Results: All radioimmunoconjugates were obtained with high radiolabeling yield (radiochemical purity, RCP > 95%) and specific activity of ca. 0.5 GBq/mg. Therapeutic effects of 177 Lu-DOTA-Rituximab were observed in animals regardless whether DOTA(SCN) or DOTA(NHS) were used for conjugation. Importantly, therapy involving 177 Lu-DOTA-Rituximab was more effective than use of Rituximab alone. Conclusions: The degree of antitumor efficacy was dependent on the type of applied bifunctional chelators conjugated to mAb. However, this difference was not statistically significant. Dosimetry calculations showed that the absorbed radiation doses extrapolated to humans were very low for osteogenic cells regardless of the conjugates. Organs like the liver and spleen, treated with 177 Lu-DOTA(SCN)-Rituximab, showed similar radiation absorbed doses when compared with 177 Lu-DOTA(NHS)-Rituximab.