The interaction between sensitized lymphocytes and specific antigen occurring in classic delayed hypersensitivity causes guinea pig peritoneal macrophages to incorporate increased amounts of glucosamine into TCA precipitable, membrane-associated, cell surface material. Antigen-induced stimulation of glucosamine also occurred in peritoneal exudate cells (PEC) isolated from animals primed for cutaneous basophil hypersensitivity with certain strong antigens (KLH, vaccinia virus) in incomplete Freund's adjuvant (IFA), and lymphocytes from such animals elaborated MIF when cultured with specific antigen. Thus, the use of complete Freund's adjuvant is not obligatory for the induction of sensitized lymphocytes capable of secreting MIF or stimulating macrophage glucosamine incorporation; however, the potency of the immunogen employed is a critical variable since lymphocytes from animals primed with weaker antigens (HSA, BGG) in IFA did not have these capabilities. Significantly enhanced incorporation of radioactive glucosamine by macrophages occurred when normal PEC were cultured in lymphokine-containing supernatants, but the magnitude of incorporation was smaller than that of sensitized PEC stimulated by antigen. The final 24 hr of macrophage culture was critically important because lymphokines were equally effective in promoting glucosamine incorporation when present for only this interval. The kinetics of this response are thus very similar to those reported for macrophage "activation". The mechanism by which sensitized lymphocytes and their products stimulate glucosamine incorporation is not established, but at least part of the increment may be attributed to enhanced transport of glucosamine across the macrophage plasma membrane. The plant lectins Con A and PHA stimulated unsensitized plastic-adherent cells to increased glucosamine in corporation and exerted a further additive stimulation on sensitized PEC when nonadherent sensitized lymphocytes were present. It is likely that these mitogens stimulate glucosamine incorporation by two distinct mechanisms, one involving sensitized nonadherent lymphocytes and a second involving only adherent cells (macrophages and/or plastic adherent lymphocytes.