Two methods of tissue culture based on gradient principles which have permitted continuous growth and multiplication of hemic cells for 6 months to 1 year are described. Three long-term cell strains were isolated from the bloods of patients with acute or chronic leukemias. Success with these methods depends on previous determination of the gradient factor for the particular cell type to be cultured. This can easily be accomplished by the gradient culture method, and these principles are in all probability applicable to all cell types grown in tissue culture.

The isolation of these cells from blood excludes a fixed tissue cell as the only type capable of long term multiplication. The development of cells resembling tissue histiocytes, Dorothy Reed cells, and Langhan’s giant cells from cells cultured from the blood of acute monocytic leukemia lends further support to the view that the monocytic series and the histiocytic series (reticuloendothelial cells) are one and the same and can give rise by polyploidy and endomitosis to many types of giant cells.

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