Osteochondrosarcoma developed in a chicken inoculated with leukosis of Strain 2. In successive passages made by intramuscular implantation of tumor tissue deriving from this chicken there occurred (a) osteochondrosarcoma free from leukosis, or (b) leukosis free from osteochondrosarcoma, or (c) osteochondrosarcoma in association with leukosis. The malignant sarcoma cells of this strain have the morphological characteristics of osteoblasts.

Implantation of tumor tissue from chickens with lesions (a) and (c) yielded both osteochondrosarcoma and leukosis. Successive passages made by intravenous and intramuscular injections with blood from chickens with leukosis yielded leukosis only. Hence it is very probable that Strain 12 consists of a mixture of an osteochondrosarcoma-producing agent and the agent of leukosis of Strain 2.

Strain 12 is readily transmitted by material containing the living malignant osteoblasts but transmission is seldom successful with virus freed from living cells. This virus produces neoplasms only when brought in contact with bone or cartilage.

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