Background: Human epidermal growth factor receptor 2 (HER2) is found to be amplified in ∼15%-20% of breast cancers. In this study, the authors report the synthesis and comparative in vitro therapeutic efficacy of 177Lu-CHX-A″-DTPA-trastuzumab and 177Lu-CHX-A″-DTPA-F(ab')2-trastuzumab to determine their potential as theranostic agents for patients with breast cancer. Materials and Methods: Bivalent F(ab')2-trastuzumab was produced by enzymatic digestion of trastuzumab, conjugated with p-SCN-Bn-CHX-A″-DTPA and subsequently radiolabeled with 177Lu. Cell viability, membrane toxicity assays, and apoptosis analysis were carried out with 177Lu-CHX-A″-DTPA-trastuzumab and 177Lu-CHX-A″-DTPA-F(ab')2-trastuzumab in HER2-positive ovarian (SK-OV-3) and breast cancer (SK-BR-3 and MDA-MB-453) cells. Results: In vitro cell binding studies showed ∼20%-25% binding of 177Lu-CHX-A″-DTPA-trastuzumab and 177Lu-CHX-A″-DTPA-F(ab')2-trastuzumab to SK-OV-3, SK-BR-3, and MDA-MB-453 cells. The cells exhibited similar degree of membrane integrity and cellular toxicity when treated with same amount (activity) of 177Lu-CHX-A″-DTPA-F(ab')2-trastuzumab and 177Lu-CHX-A″-DTPA-trastuzumab, and the toxicity was dose dependent. The mode of cell death was predominantly by apoptosis and necrosis with both the radioimmunoconjugates. Conclusions: The results indicated that the efficacy of both the radioimmunoconjugates, in terms of inducing cell death, was similar thereby ascertaining their potential as good therapeutic agents for patients with breast cancer.