Electron microscope studies of eight different sublines of Ehrlich ascites tumor cells which had not, as far as could be determined, come in contact with any known virus, revealed dense particles measuring approximately 55 to 70 mµ in diameter, both within and attached to the wall of cytoplasmic vesicles identified as the endoplasmic reticulum.

All tumor sublines contained significant numbers of particles and revealed no qualitative or quantitative differences in particle morphology or distribution.

It is concluded that these structures are a constant morphological component of the Ehrlich ascites tumor and that they probably do not represent contaminating virus. Their morphology and distribution are described, and the possible interpretations of their significance are discussed.

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