An agent was cultivated in tissue culture from avian lymphomatous liver filtrates which can be passed serially in chick embryo liver cells and which induces cytopathological changes. The agent induces in 12-day-old chick embryos a disease histologically resembling lymphomatosis. In chicks it has induced lymphomatosis in 20% to 55% of the birds in 9 months, and an agent similar to the original agent was recovered from some of the diseased livers.