Clearing agents are often used in pretargeting despite the potential for decreased tumor accumulation of the effector. However, according to the authors' semiempirical model, a clearing agent should not necessarily decrease tumor accumulation. In this study, the authors have added a clearing step to their model-morpholino phosphorodiamidate oligomer (MORF)/complement MORF (cMORF) pretargeting system-to confirm this prediction. The CC49 antibody was conjugated with both biotin and an 18 mer MORF. The influence of avidin on antibody clearance was first evaluated in normal mice in which each animal received 30 mug of MORF-CC49-biotin, 0-70 mug of avidin 1 day later, and 1.2 mug of (9)(9)(m)Tc-cMORF 3 hours later, with sacrifice at 3 hours. Thereafter, a pretargeting study in mice bearing an LS174T tumor was performed at a 34 mug avidin dosage. In normal mice, the blood level of (9)(9)(m)Tc-cMORF fell by 60% at an avidin dosage of 10 mug or higher. In tumored mice, avidin produced a similar reduction in blood but had no influence on tumor level, which remained at 6.30% ID/g as predicted. In conclusion, in addition to the expected reduced effector levels in blood and normal tissues, a reduction in tumor accumulation was avoided when adding a clearing agent as predicted.