BACKGROUND Preinjection of a nonlabeled tumor targeting anticytokeratin monoclonal antibody (TS1) and postinjection of an anti-idiotypic anticytokeratin monoclonal antibody (alphaTS1) were evaluated separately and in combination to investigate their effects on the accumulation of iodine-125 (125I)-labeled TS1 in experimental radioimmunotargeting. TS1 targets deposited extracellular cytokeratin 8 from necrotic tumor cells. METHODS Nude mice were inoculated with HeLa Hep 2 cells. Four different groups were followed with 504 repetitive quantitative radioimmunoscintigraphic recordings during a 78-day observation period. The absorbed doses were calculated according to criteria of the Medical International Radiation Dose Committee. RESULTS As much as 2% of the injected dose (ID) of 125I-labeled TS1 accumulated in the tumor, and the peak tumor uptake was recorded as late as Day 30 after the injection of 125I-labeled TS1. Anti-TS1 caused a rapid decrease in the whole body activity. The highest tumor-to-nontumor activity ratios were obtained when a pre-injection of nonlabeled TS1 was combined with a postinjection of alphaTS1. The mean absorbed dose in tumor per unit activity administered was 0.44 gray/megabecquerel (Gy/MBq) and in nontumor tissues 0.15 Gy/MBq after a single injection of 125I-TS1. The efficacy was 0.34 Gy/MBq in tumor and 0.1 Gy/MBq in nontumor tissues after a combination of preinjection of nonlabeled TS1 and postinjection of nonlabeled alphaTS1. This indicates a 20% increase in tumor doses compared with a single injection of labeled TS1. CONCLUSIONS This study confirms an extensive accumulation of TS1 in the tumor, with peak values as late as 30 days after injection of labeled TS1. Furthermore, both preinjection of nonlabeled TS1 and postinjection of alphaTS1 can improve radioimmunotargeting.