Forty-eight cases of leukemia were identified in 629 control and irradiated guinea pigs observed at the National Cancer Institute during the years 1947 through 1960. Twenty-eight of the cases were lymphatic and 20 were stem cell leukemia. Statistical evidence for radiation induction of lymphatic and stem cell leukemia was found for special conditions of limited, chronic gamma-ray exposure. Stem cell leukemia was seen only in irradiated animals. In eight of the cases, including both types of leukemia, transplantation of the disease to normal syngeneic pigs was demonstrated. Unusual "reactive changes" in lymphatic tissues were seen at autopsy in five animals with stem cell leukemia. The pathology findings suggest that lymphatic leukemia originates in lymph nodes and stem cell leukemia in the bone marrow.

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