AbstractMurine lymphoid cells which were stimulated in liquid culture containing thymus culture fluid (Thy‐CF) and seeded in a soft agar culture system, proliferated and developed into B cell colonies. Two types of colonies were formed: large colonies within the upper layer and small flat colonies on the surface of the upper layer.Thy‐CF prepared from cells of normal hydrocortisone‐treated mice had a higher cloning potential than Thy‐CF prepared from untreated mice. At concentrations of Thy‐CF in culture medium greater than 35, Thy‐CF prepared from normal mice had an inhibitory effect on colony formation.Cells of nude mice were also able to form B cell colonies if thymocytes of normal mice were mixed with lymphoid cells in the culture medium. Thymocytes elaborate a B lymphocyte colony‐stimulating factor which, with the help of T cells, triggers a B cell population into colony formation and immunoglobul
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