The appearance of cellular associations between macrophages and lymphocytes--which we have denominated macrophage-lymphocyte rosettes--and their kinetic formation in the presence of phytohemagglutinin (PHA) have been studied in B10 A (4R) mice. The greatest number of macrophage-lymphocyte rosettes was found from 6 to 12 hours after incubation with PHA. During this time, 42.38 +/- 10.70 of the total number of macrophages had lymphocytes attached to their membranes. This percentage decreased to 17.33 +/- 2.07% after 24 hours. The activation of macrophages after PHA treatment was tested by the phagocytic capacity of these cells. This activity increased significantly 24 hours after incubation. In our assay, an increase in the appearance of multinucleated giant cells when compared to controls was also observed. When the macrophages were lymphocyte depleted, the appearance of the multinucleated giant cells was significantly lower. The kinetics for these formations are also discussed.