Histology and ultrastructure of a sebaceous adenoma of the mouse -- derived from i.p.-injection of 0.1 ml DNA, isolated from cells of a transplantable mouse-leukemia -- were studied. The original tumour and all transplanted tumours display the same tissular differentiation. In all tumours virus particles were found. Structure and behaviour of the intracytoplasmic A-particles resemble those of the mouse mammary tumour virus while the C-particles found extracellular were identical with the leukemia viruses of the mouse. It might be suspected that mammary tumour virus and leukemia viruses are either non specific or that a new virus is present in the sebaceous adenoma.