Alzheimer's disease (AD) is characterized by amyloidosis of brain tissues. This phenomenon is studied with genetically-modified mouse models. We propose a method to quantify amyloidosis in whole 5xFAD mouse brains, a model of AD. We use optical projection tomography (OPT) and a random forest voxel classifier to segment and measure amyloid plaques. We validate our method in a preliminary cross-sectional study, where we measure 6136 +/- 1637, 8477 +/- 3438, and 17267 +/- 4241 plaques (AVG +/- SD) at 11, 17, and 31 weeks. Overall, this method can be used in the evaluation of new treatments against AD. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement