There is provided an in vitro method for expansion of hematopoietic cells and hematopoietic cell progenitors comprising: culturing bone marrow or peripheral blood cells with a composition comprising an amount of IL-21 sufficient to produce an increase in the number of lymphoid cells in the bone marrow or peripheral blood cells as compared to bone marrow or peripheral blood cells cultured in the absence of IL-21. There are also provided anti-cancer effector cells for use in the treatment of cancer in a patient by adoptive immunotherapy, wherein said anti-cancer effector cells have been obtained from the patient, and wherein the use comprises an initial ex vivo step of expanding the anti-cancer effector cells in culture in the presence of IL-21.