Background: Monoclonal antibodies have confirmed their merit as biotherapeutics across a wide spectrum of diseases, including cancer, heart disease, infection, and immune disorders. Materials and Methods: The dynamics of Zr-89-labelled monoclonal antibodies (MAb) after injection into the human body are modelled. This modified biokinetic model can be used for dose assessment not only for Zr-89-labelled MAb tumour visualization but also for diagnostic and radiation therapy with other MAb-labelled radionuclides. The created modified biokinetic model is based on experimental data from the literature. The cumulative Zr-89 activity in organs and tissues per Bq of administered activity is calculated with the WinAct program. Results: For the organs receiving the highest radiation exposure, the average absorbed doses were estimated with IDAC 2.1 software. The results from the modelled calculations are compared with new published experimental diagnostic results from real patients. The calculations reveal that the organs which received the highest dose were the spleen, liver, kidneys, and red bone marrow (doses of 1.54, 1.33, 0.81 and 0.82 mGy/MBq, respectively). In the modified biokinetic model, the organs exhibiting the highest dose were the liver, gallbladder wall, spleen, pancreas, and kidneys (at 1.07, 0.77, 0.70, 54 and 0.44 mGy/MBq, respectively), when the injection was associated with monoclonal antibodies. Conclusion: The developed biokinetic model is in good agreement with direct measurements and can be considered a first step for simulating the radiopharmaceutical dosimetry of the Zr-89 isotope.