Corticosteroid action on rat spleen tissue was studied in order to determine the action of the hormone on cells involved in the early phase of the immune response. A significant reduction of plaque forming cells (PFC) was observed when the steroid was administered simultaneously with the antigen and even had some effect when injected up to three days immunization. Inasmuch as it can be assumed that at a period of 72 hr or less a great number if not all the cells are challenged by the antigen, results indicate a lympholytic action of the hormone on the already stimulated cells.