In this review, we have selected some parameters with the potential to improve the efficacy of RIL and RIT. Focus has partially been on the behaviour of radiolabelled antibodies in vivo in relation to properties and amounts of both target antigen and the antibodies used. If, out of the 28 factors listed in Table 1, some should be given preference in future work, it is our opinion that after the initial saturation of the tumour site a rapid decrease in redundant antibody is of significant importance. Furthermore, quantitative aspects of both antigens and antibodies should be more carefully evaluated when possible. By combining several of the listed approaches toward increasing efficiency, a more extensive use of RIL and RIT could be expected in the future.