During a period of 2 years 32 spontaneous malignant tumors of the reticuloendothelial system were detected in a non-inbred colony of approximately 500 golden hamsters. Several tumors proved transplantable, and at present 3 tumor lines are maintained in homologous hosts. The spontaneous tumors consist of a mixed cell population ranging from atypical reticulum cells to atypical lymphocytes and plasma cells. Many elements are believed to represent transitional stages of the two main cell types. In the course of successive transplantation generations the atypical lymphocytes and plasma cells are gradually lost and the tumors become simple reticulum cell sarcomas. The cells of the tumor transplants retain the functional capacity of reticulum cells formation of reticulin fibres and phagocytosis of erythryocytes and cellular fragments. In one animal the transplant showed the full differentiation of a fibrosarcoma. In spontaneous as well as in transplanted tumors formation of capillary-like vessels by the tumor could be observed. This peculiarity, combined with prominent erythrophagocytosis, evokes the appearance of a hemangiothelioma. The same type of tumor in man has been classified by v. Albertini as angioplastic reticulum cell sarcoma.