Recombinant human migration inhibitory factor (MIF), isolated through functional expression cloning in COS-1 cells, up-regulates expression of genes encoding HLA-DR and interleukin 1 beta (IL-1 beta) and elaboration of IL-1 beta by human monocyte-derived macrophages. Administration of soluble bovine serum albumin or human immunodeficiency virus 120-kDa glycoprotein (HIV gp120) to mice in the presence of recombinant MIF together with incomplete Freund's adjuvant induced a strong T-cell proliferative response comparable to that of complete Freund's adjuvant. Recombinant MIF also increased antibody production, especially of IgG1 and IgM, in mice. Taken together, these results indicate that recombinant MIF may be useful as an adjuvant in the development of vaccines.