Our objective was to compare the cell penetration and nuclear importation properties of 111 In-labeled and 123 I-labeled immunoconjugates (ICs) composed of 16-mer peptides (GRKKRRQRRRPPQGYG) derived from HIV-1 transactivator of transcription (tat) protein and anti-mouse IgG (mIgG) in BT-474 breast cancer (BC) cells.[ 111 In]tat ICs were constructed by site-specific conjugation of tat peptides to NaIO 4 − -oxidized carbohydrates in the Fc domain of diethylenetriaminepentaacetic-acid-modified anti-mIgG antibodies. Immunoreactivity against mIgG was assessed in a competition assay. The kinetics of the accumulation of [ 111 In]anti-mIgG-tat IC and [ 123 I]anti-mIgG-tat ICs in BT-474 cells and the elimination of radioactivity from cells, cytoplasm or nuclei were determined. The effects of excess tat peptides or NH 4 Cl (an inhibitor of endosomal acidification) on cellular uptake and nuclear importation of [ 111 In]anti-mIgG-tat were measured.[ 111 In]anti-mIgG-tat was >97% radiochemically pure and exhibited preserved immunoreactivity with mIgG epitopes. [ 123 I]Anti-mIgG-tat penetrated BT-474 cells more rapidly than [ 111 In]anti-mIgG-tat ICs and achieved a 1.5-fold to a 2-fold higher uptake in cells and nuclei. Cell penetration and nuclear uptake of [ 111 In]anti-mIgG-tat were inhibited by excess tat peptides and NH 4 Cl. Elimination of radioactivity from BT-474 cells and nuclei was more rapid and complete for 123 I-labeled than for 111 In-labeled anti-mIgG-tat ICs.Tat peptides derived from HIV-1 tat protein promoted the penetration and nuclear uptake of radioactivity following the incubation of 111 In-labeled and 123 I-labeled anti-mIgG antibodies with BT-474 human BC cells. 111 In-labeled tat ICs are feasible for inserting radionuclides into cancer cells with potential for targeting intracellular and, particularly, nuclear epitopes for imaging and/or radiotherapeutic applications.