In the course of the study of cancer cells in the blood stream of hamsters with grafted tumors, different forms of megakaryocytes were observed. The morphology and occurrence of these cells are presented. The cells of the megakaryocyte line can be easily mistaken for tumor cells, especially the uncommon form. A correlation was found between an increase of megakaryocytes, including their atypical forms, and the occurrence of an extramedullary reaction. This reaction was present in 65 of 102 hamsters with polymorphous metastatic sarcoma. Myeloid reaction was observed more frequently in the liver than in the spleen or lymphatic nodes and in animals with metastases than in those with none. This reaction is not characteristic for malignant diseases and its significance is discussed.

A study of the cytological and ultrastructural characteristics, chromosomal value and tumorigenesis of a cultivated golden hamster (Mesocricetus auratus) malignant melanoma was made. Four kinds of cells were observed in the cultures: melanocytes, fusiform cells and two forms of macrophages. Electron microscope observations showed very irregular nuclei with intranuclear lesions, in a type of ring‐like bodies. Vast Golgi zones with melanin formation were seen in the cytoplasm. The melanosomes presented a laminar formation. Extracellular virus‐like particles, 80 mμ in diameter, were observed occasionally. The modal value of chromosome numbers was 44 and they ranged from 42 to 270. When inoculated in hamsters, as small as 500 cells/ml developed tumors in 44–60 days with metastases which retained the property to produce melanin. The significance of the nuclear lesions and virus‐like particles in the golden hamster melanoma cells remain open for discussion.

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