Radioactively labeled antibodies have been developed (by others) which localize to tumors sufficiency to be used for radiation therapy. However, it is essential to know the absolute uptake of radioactivity in order to accurately compute dosimetry. There are two corrections required in order to accurately quantitate the absolute uptake of activity: scatter correction and attenuation correction. In the first phase of this project we have concentrated on attenuation correction of planar images. Theoretical studies were performed to assess the magnitude of inaccuracies which would result if uptake was inhomogeneously distributed, or even homogeneously distributed over a large volume. Experimental studies indicated that so called "conjugate view" imaging combined with collimated transmission source measurements could give quite accurate quantitative results. A studies were undertaken to determine the optimum collimator design, and to determine how best to scale low energy (Tc-99m) measurements to the higher energies often used for antibody imaging. These scaling studies indicated one could perform attenuation correction with the easily obtainable Tc, and apply the data quite accurately to higher energies. Future work will be undertaken to include a more accurate scatter correction and to extend the work to tomographic imaging.