Although subject to limitations, there is a need for carefully controlled laboratory studies using animal tumor models in research on tumor-localizing agents. This paper reviews the literature relating to the more important transplantable tumor systems, spontaneous or induced, as to origin, host, site, and radioactive agent used. The historical background of animal tumor models is discussed, including such technical aspects as source of tumor, techniques of transplantation, transplantation sites, and maintenance of transplants. Also, considered are the use of animal tumor models as predictive systems, expression of experimental results of quantitative studies of tissue uptake and comparative radionuclide tumor and distributional studies, and suggestions for future studies, such as the need of more intensive study of existing tumor models for a better understanding of the relationship between animal and human tumors, the need for the development of new tumor model systems, and for standardization of experimental protocols and procedures. A total of 48 t,mor models (26 in mice, 11 in rats, 7 in hamsters, 2 in rabbits, and 2 in dogs) are presented in seven tables dividing the models into tumors of epithelial tissue, connective tissue, hematopoietic tissues, melanin-forming tissue, neural tissues, undetermined site of origin or undifferentiated histologic pattern, and miscellaneous background. The use of animal tumor models in cancer research, which utilizes radionuclides, permits the investigator to do many things not permissible with human beings, but the extrapolation of animal results to human beings must be approached with caution, Although malignant disease, whether in animals or man, must be individualized, certain trends in animal studies can be observed. It is the goal of the laboratory investigator to be able to indicate to the clinician those trends or phenomena that, when repeatedly observed in animal model systems, may be applicable to an understanding of malignant disease in man.