In radioimmunotherapy, the emission characteristics of the radioisotope is critical in determining the radiation dose to the tumor compared to normal organs. If antibodies internalize and transport low-energy electron emitting isotopes close to the tumor cell nucleus, an improved therapeutic advantage is achieved. METHODS: Using fluorescent microscopy, we studied the subcellular distribution of an internalizing antibody, A33, which detects a restricted determinant on colon cancer cells. We developed a physical model to assess the dose deposited on the nucleus by electrons emitted from radiolabeled A33 accumulated inside vesicles. This model is based on the energy-range relationship of electrons in water. Similarly, another model was developed to calculate the radiation dose to the nucleus from electrons emitted from extracellular space. The percentage of A33 bound to membrane and internalized was determined in vitro at various time points. Cytotoxicity experiments were performed with 125I- and 131I-labeled A33 at various concentrations and specific activities. RESULTS: A33 accumulates in cytoplasmic vesicles (40% of total bound) which transport the activity close to the nucleus. This increases the radiation dose to the cancer cell nucleus by a factor of 3 compared to the average dose calculated based on the assumption of a uniform distribution on the cell membrane. The cytoplasm of antigen-negative normal cells shields the nucleus from the electrons emitted from extracellular 125I. This shielding is 30 times less for 131I. Cytotoxicity data show 10% cell survival with 10 microCi/ml of 125I-A33, but 90% survival with up to 100 micro/Ci/ml of 125I-A33 in the presence of a blocking dose of 100-fold excess of cold A33. Similar experiments with 131I showed cytotoxicity in both cases. CONCLUSIONS: The results of the cytotoxicity experiment are in agreement with the physical model and suggest a basis for improved tumor-to-marrow radiation dose by clinical use of 125I-A33.