To study the role of follicular dendritic cells (FDC) in germinal centers, we tried to keep them alive in vitro by culturing entire lymphoid follicles. Ultrastructural studies of those cultures in different conditions of culture technique, medium and temperature have been made. In any considered conditions living FDC were not found in the cultures after the 7th day. But during that period, only lymphocytes in close contact with the cytoplasmic processes of FDC survived. FDC seem thus to exert a positive action on lymphocyte survival in vitro. Moreover, FDC and lymphocytes appeared associated in situ in clusters similar to those obtained after isolation of FDC (Lilet-Leclercq et al., J. Immunol. Meth., 1984, 59, 235).