The X5563 tumor has been grown in tissue culture. Cells similar to those of the original tumor migrated from the explant and attached to the glass walls of the culture vessels. Electron microscopy showed that large numbers of particles, similar in morphology to virus particles, were associated with these cells after 7 days of culture. The two principal types of particles found in the tumor in vivo appear to be present in vitro. Many more of these particles, however, were larger and showed a more complex structure. Whereas the particles were mainly localized inside endoplasmic reticulum or the Golgi zone in the tumors in vivo, in the tissue culture the majority of the particles were associated with the plasma membrane and were found outside of the cells. The relation of the particles to the granular body is discussed as well as a possible relation to the mammary tumor agent.

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