In common with other neoplasms, the Rous No. 1 tumour often shows small areas or even single cells that appear dead and disintegrating or moribund. These probably result from the interaction of such factors as inadequate vascularization, pressure, and the formation of non-viable cells as a result of abnormal mitosis. As death of the cell does not necessarily result in destruction of the tumour virus, the virus of such cells is presumably released into the host's tissues. There is some difference of opinion as to the part that this virus plays in extending the growth of the tumour by infecting neighbouring cells and by producing metastases (review in Foulds, 1934), but it is generally agreed that the virus is present in most tissues of an animal bearing a filterable tumour (Bürger, 1914; Fujinami and Suzue, 1925; Fränkel, 1927; Costa, 1932; Mellanby, 1935, 1936, 1937, 1938 a, 1938 b) without, however, producing any tumours. The reason for this absence of carcinogenic action on the part of the disseminated virus has remained obscure. It was also noted that recurring Rous tumours are always found at the sites inoculated with tumour material, and never result from disseminated virus (Carr, 1942). Mellanby (1938 b) noted that the amount at first increased and later decreased with time, no active virus being found forty or more days after the initiation of tumour growth in the host. A similar decrease in the amount of extractable virus in tumours has been described (Carr, 1943) and shown to be due to antibodies in the tissue fluid inactivating the virus as it is extracted from the tumour cells. This raised the possibility that virus was similarly present in the normal tissues of hosts bearing lasting tumours, though not demonstrable for the same reason. The present investigation was undertaken to see if this was the case, and to discover the reason for the absence of neoplastic action of such virus.