Abstract The in vitro induction of cytostatic/cytotoxic activity in macrophages generated in spleen cell cultures requires a signal cascade initiated by costimulation with both LPS and IFN-gamma. Th2 lymphocytes, although they do not produce IFN-gamma, can provide the signals necessary for induction of cytostatic activity in IFN-gamma-primed macrophages. These signals appear to be delivered by cognate interaction between the Th2 cells and macrophages in that: 1) they are not delivered by culture supernatants of Th2 cells activated 6 or 20 h by Con A or by immobilized anti-CD3 mAb; 2) they are not delivered if cell contact between Th2 cells and macrophages is prevented; and 3) they can be delivered by paraformaldehyde-fixed activated Th2 cells. Paraformaldehyde-fixed resting Th2 cells cannot stimulate activation of INF-gamma-primed macrophages. The Th2 cells must be activated at least 3 h before fixation to acquire macrophage stimulatory activity. Optimal macrophage-stimulating activity is attained after 6-h activation of the Th2 and declines thereafter. Although the activation of IFN-gamma-primed macrophages by viable resting Th2 displays Ag specificity and MHC restriction, the activation of IFN-gamma-primed macrophages by paraformaldehyde-fixed activated Th2 is neither Ag specific nor MHC restricted. These observations suggest that T cell-mediated activation of macrophages can involve a signaling cascade of Ag-specific and Ag-nonspecific adhesion events comparable to those hypothesized to occur in T cell-mediated B cell activation.