In the five patients in this study a preload of rituximab improved the biodistribution of the radioconjugate by preventing its sequestration in patients with preserved circulating CD20+ lymphocytes, but consistently impaired radioconjugate tumour targeting in patients with B-cell depletion, the latter representing the majority of patients eligible for RIT in the “rituximab era”. Here, MS4A1 is linked to neoplasm.