Enhancement of tumor induction in rats with Moloney murine sarcoma virus by a "new" method based on direct injection into fetuses.

Undiluted, fivefold-diluted, and 25-fold-diluted doses of a stock of Moloney murine sarcoma virus were injected directly, in a volume of 0.025 ml, into the backs of fetal Sprague-Dawley rats by laparotomy through the uterine wall at 18 days of gestation. During the first 8 weeks after birth the young responded to the virus with remarkably high but dose-dependent incidences of neoplasms. When a one-fifth dilution of the virus preparation was inoculated at fetal ages 16, 18, and 20 days, the incidences of lesions decreased with advancing fetal age. The tumors developed preferentially at the virus inoculation site and/or in the proximal parts of the extremeties; all were considered to be of mesenchymal derivation, i.e., malignant mesenchymoma, rhabdomyosarcoma, osteosarcoma, fibrosarcoma or fibromyxosarcoma, hemangiosarcoma, plasmacytoma, and a giant cell tumor. This injection procedure provided us with a valuable experimental tool for the rapid screening or testing of potential chemical carcinogens and other biologic studies.