The M25 monoclonal antibody reacts with mouse and human IL-7, a 20-28 kDa hematopoietic growth factor secreted by stromal cells in the bone marrow and thymus. IL-7 is also produced by keratinocytes, dendritic cells, hepatocytes, neurons, and epithelial cells. IL-7 binds to the IL-7 receptor, a heterodimer consisting of IL-7 receptor alpha and the common gamma chain receptor. Binding results in stimulation of differentiation of pluripotent hematopoietic stem cells into lymphoid progenitor cells and proliferation of all lymphoid cells. IL-7 is also important for proliferation of pre-B cells as well as T and NK cell survival, development and homeostasis. The M25 antibody is useful for neutralizing the bioactivity of IL-7 as well as administration of IL-7 cytokine. IL-7 administration as a complex with M25 prolongs IL-7 availability in vivo by decreasing specific and nonspecific consumption.