Moloney murine sarcoma virus (M-MSV) was injected directly into the fetuses of Sprague-Dawley rats during the late stage of gestation and into the neonates within 24 hours after birth. Ninety rats developed 188 neoplastic lesions during the 8-week period of observation. Nearly all of the neoplasms were of mesenchymal derivation. Sixty percent of these neoplasms revealed more complex histologic features than those previously reported for neoplasms induced in rodents with M-MSV and were designated "malignant mesenchymoma" which developed preferentially in the proximal parts of the extremities, distant from the inoculation site. Rhabdomyosarcoma and osteosarcoma which developed in a pure form at the various sites were the next most common tumor type. Osteosarcoma developing in a pure form and as a component of malignant mesenchymoma in the humerus and femur was comparable to that of juxtacortical osteosarcoma of man; The development of excessive bones were observed in the forelimb and/or hind leg, suggesting a type of skeletal malformation. The reaction to M-MSV merits attention as a model for the study of an osteosarcoma and malignant mesenchymoma as well as rhabdomyosarcoma and also for the study of viral teratogenesis in man, as "rubella syndrome".