Three-dimensional dose distributions have been calculated for LS174T human colon cancer xenografts in athymic nude mice treated with 131I-labeled 17-1A monoclonal antibody. Autoradiographs were made for fifteen to twenty 32-micron-thick representative serial sections of tumors removed 1 and 4 days postinjection. Film density readings were converted to activity density and entered into a radiotherapy treatment planning system. Three-dimensional dose distributions were obtained by summing the dose contributions due to each voxel of uniform activity. Isodoserate distributions and dose-rate-volume histograms for representative tumors at 1 and 4 days following 131I-labeled 17-1A injection showed a progressive change from a predominantly surface deposition (day 1) to a more volumetric deposition (day 4). Average tumor doses calculated using the assumptions of uniform source distribution and local dose deposition resulted in a poor estimation of the cumulative dose because of the significant time-dependent dose-rate nonuniformities.