One-way mixed-lymphocyte reactions (MLR) were performed to determine qualitatively and quantitatively how macrophages and macrophage-derived factors (MDF), interacting with mouse spleen nonadherent (NA) subpopulations, affected the ability of NA cells to respond to allogeneic lymphocytes. Comparative reactivity was measured using normal cells and cells from mice in various stages of tumorigenesis. In MLR's using normal NA cells, recognition of allogeneic cells was dependent on addition of an optimal (2-3%) concentration of macrophages. Beyond this optimum concentration, macrophage addition became inhibitory to NA cell stimulation. Macrophage enhancement or inhibition was not contact dependent, since cell-free macrophage supernatants (depending on their concentration) also possessed a similar capacity for enhancement or depression of the thymus-derived (T) cell response to allo-antigens. In the tumor-bearing host, initial phases of tumorigenesis caused in vitro NA cell activation in the absence of macrophages or MDF; however in advanced stages of tumorigenesis, NA cells required macrophages in order to respond to histoincompatibility antigens. Macrophages and MDF from normal or one week palpable tumor-bearers did not differ in their ability to enhance or depress in vitro T cell activity. Macrophages and MDF from two-week palpable tumor-bearing hosts exhibited a decreased capacity to stimulate. As tumorigenesis progressed, MLR activity of the NA population decreased, in response to the suppressor action of macrophages and macrophage supernatants. To explain the dual role of the macrophage as both an enhancing cell and a repressor cell, possession of two macrophage supernatant factors has been proposed. / Master of Science