Murine 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 immunoglobulin production.