In what we be believe to be the first report of the use of 89Zr-rituximab, the aim of this study was to compare the distribution of 89Zr-rituximab with and without a standard preload of unlabelled rituximab in patients with relapsed CD20+ B-cell lymphoma, with the aim of assessing the potential impact of circulating anti-CD20 antibodies on whole-body distribution, radiation dose and tumour targeting of a subsequent radiolabelled anti-CD20 antibody as part of a RIT regimen. Here, MS4A1 is linked to B-cell non-Hodgkin lymphoma.