Highly conserved DBP (human DBP is known as Gc) of serum alpha 2-globulin fraction can be converted to a potent macrophage activating factor by stepwise modification of Gc glycoprotein with beta-galactosidase of B cells and sialidase of T cells. These glycosidases, beta-galactosidase and sialidase, are membrane bound and not soluble in culture medium. Thus, consecutive contact of Gc protein with B cells and T cells, presumably via specific receptors, is required for conversion of Gc glycoprotein to the macrophage activating factor. The essential role of T cell sialidase in macrophage activation was confirmed by the finding that peritoneal nonadherent cells of SM/J mouse, whose T cells are deficient in sialidase activity, were unable to convert Gc protein to the macrophage activating factor and thus did not activate macrophages. Treatment with sialidase of a conditioned medium of lipid metabolite-treated SM/J mouse nonadherent cells efficiently generated the macrophage activating factor. When Gc protein was first treated with soluble or immobilized sialidase and used in a medium for 2 h cultivation of lipid metabolite-treated SM/J mouse nonadherent cells or BALB/c mouse B cells, the resultant conditioned media contained a large amount of the macrophage activating factor. These results support the hypothesis that Gc protein carries a dibranched trisaccharide with galactose and sialic acid termini.