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Pigmented melanomas were induced in the Syrian golden hamster by oral administration of 9, 10-dimethyl 1,2-benzanthracene. The primary tumors were grafted into new animals to obtain a series of transplantable tumors. The grafted tumors show the malignant characters which are not present in the primary tumors. These were studied clinically, histologically and ultrastructurally. We believe that this transformation occured perhaps as a consequence of the selection of cells which have a more prominent tendency toward malignancy. This selection could be explained by a greater resistance of lesser pigmented melanocytes or by their weak immunologic capacity. The malignant potential of these cells could eventually be correlated with the presence of viral particles which act as cocarcinogens.