Immunological functions were analyzed in mice lacking granulocyte/macrophage colony-stimulating factor (GM-CSF). The response of splenic T cells to allo-antigens, anti-mouse CD3 mAb, interleukin 2 (IL-2), or con-canavalin A was comparable in GM-CSF +/+ and GM-CSF –/– mice. To investigate the responses of CD8+ and CD4+ T cells against exogenous antigens, mice were immunized with ovalbumin peptide or with keyhole limpet hemocyanin (KLH). Cytotoxic CD8+ T cells with specificity for ovalbumin peptide could not be induced in GM-CSF –/– mice. After immuni-zation with KLH, there was a delay in IgG generation, particularly IgG2a, in GM-CSF –/– mice. Purified CD4+ T cells from GM-CSF –/– mice immunized with KLH showed impaired proliferative responses and produced low amounts of interferon-γ (IFN-γ) and IL-4 when KLH-pulsed B cells or spleen cells were used as antigen presenting cells (APC). When enriched dendritic cells (DC) were used as APC, CD4+ T cells from GM-CSF –/– mice proliferated as well as those from GM-CSF + / + mice and produced high amounts of IFN-γ and IL-4. To analyze the rescue effect of DC on CD4+ T cells, supernatants from (i) CD4+ T cells cultured with KLH-pulsed DC or (ii) DC cultured with recombinant GM-CSF were transferred to cultures of CD4+ T cells and KLH-pulsed spleen cells from GM-CSF –/– mice. Supernatants from both DC sources contained a factor or factors that restored proliferative responses and IFN-γ production of CD4+ T cells from GM-CSF –/– mice.