Tumours induced in chickens by inoculation of avian sarcoma viruses are frequently capable of undergoing spontaneous regression. It is only those tumour cells which have been derived from progressively growing neoplasms that are able to produce transforming progeny virus in vitro and to shed into the culture medium antigens which are specifically reactive with the peripheral lymphocytes of sarcoma-bearing hosts. Following multiple passages and extended growth in culture, however, the ability of these tumour cell fluids to stimulate the lymphocytes of sensitized hosts diminishes in concert with the declining capacity of these cells to continue to synthesize fully transforming progeny virus. In certain instances, however, aged tumour cells are able to synthesize particles which contain the enzyme RNA-dependent DNA polymerase yet lack detectable envelope glycoprotein.