Half an hour, 4, 8, 16, 24 hours and 6 days after the injection of chicken red blood cells (CRBC) into the hind foot pads of germfree mice their popliteal lymph nodes were studied in one-micron sections and in the electron microscope. The untreated germfree lymph node was characterized by the absence of a sinus system and a poor cellularity. Mast cell degeneration, dilatation of small blood vessels, infiltration of the peripheral lymph node parenchyma with polymorphonuclear cells and formation and widening of a sub-capsulary sinus space were early signs of antigenic stimulation. Later, dilatation of high endothelial venules filled up with leucocytes, lymphocyte infiltration and lymphocyte degeneration were prominent features of the induction phase. Six days after stimulation germinal centre activity had developed.