Chick embryo fibroblasts infected with Rous sarcoma virus in vitro are rendered malignant for such cells produce typical Rous sarcomas when injected into susceptible chicks since the tumors produced predominantly retain the sex chromatin patterns of the donor cells when such cells are injected into a recipient of the opposite sex. However, examination of the sex chromatin of cells at the periphery of the tumor shows presence of recipient cells though the bulk of the tumor is clearly of donor cell origin. Such tumors grow and cause death of the recipient. Injection of RSV induces tumors of the sex of the recipient as also does the injection of transformed cells rendered incapable of multiplication by x-rays. Following their injection into susceptible chicks, the cells transformed in vitro by virus behave in the same manner as tumor cells obtained from tumors induced by virus in vivo and cultivated in the same conditions in vitro.

When such tumors induced by transformed cells are serially transferred in recipients of the opposite sex, they gradually convert to the sex of the recipient indicating that the tumors are not indefinitely transplantable.

These chick embryo fibroblasts transformed in vitro show the same neo-plastic properties as tumor cells when they are introduced into the cheek pouch of the hamster or the eye of the guinea pig.

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