Hamsters inoculated with murine sarcoma virus (Moloney) of mouse or rat origin developed nonregressing, metastasizing, undifferentiating sarcomas.

Light microscopy and ultrastructural studies of the hamster sarcoma revealed less tumor-cell differentiation than reported for the mouse or rat.

Cell-free extracts prepared from the hamster tumors were oncogenic only in animals of the same species. All attempts to induce the sarcoma in the mouse or the rat by the hamster tumor cell-free preparations were unsuccessful. Preliminary attempts to demonstrate focus-forming activity in cell-free tumor extracts in vitro using cells of mouse, rat, or hamster origin have been unsuccessful.

In vivo and in vitro rescue of the murine sarcoma virus genome was possible from hamster tumor cells by cocultivation with mouse cells and murine leukemia virus.

The hamster tumors were readily transplantable, and, in contrast to the induced tumors in the mouse or rat, no regression occurred through 27 whole-cell transplant generations. Malignancy of the hamster tumors increased progressively through the transplant generations or through consecutive cell-free passage as manifested by less prolonged survival times and less tumor-cell differentiation of the recipient host.

Although virus particles were not found on thin sections of the tumor, a few C-type particles were evident in tumor cell-free concentrates by employing the negative staining procedure.

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