In the first part of experiments the GvHR activity of the spleen cell suspensions from normal parental donors (Balb/c) or infected with Listeria monocytogenes was compared. GvHR was examined in (Balb/c X AKR) F1 mice. Full suspensions from these donors or depleted of adherent cells or with addition of macrophages were studied. It has been proved that adherent cells population plays a significant role in the GvHR development. The addition of syngeneic macrophages from F1 hybrids results in a greater augmentation of GvH reactivity of parental lymphocytes. However, the addition of macrophages harvested from F1 hybrids immunized with L. monocytogenes 7 days before, brings about a weaker GvH reaction. Spleen cells of parental donors injected with L. monocytogenes 5-6 days earlier induced stronger GvHR as compared with the splenocytes of normal donors. Similarly, F1 hosts treated with L. monocytogenes 3-5 days before injection of normal donor's spleen cells showed increased GvHR, but those infected 7-9 days before injection of spleen cells, developed only very weak GvHR.