The subcutaneous administration of cortisone acetate to C3H and AKR mice was followed by thymic atrophy, decreased mitotic activity in thymic lymphoid cells and a selective destruction of thyimic small lymphocytes. During the succeeding regenerative phase, mitotic activity was higher than in control thymuses and was concentrated around the PAS‐positive reticulum cells of the cortex. Following adrenalectomy, thymic lymphocyte mitotic indices rose and the weight of the thymus increased. The increased mitotic activity was again concentrated around the cortical PAS‐positive reticulum cells.

These findings are further evidence indicating that PAS‐positive reticulum cells are involved in the regulation of lymphocyte mitotic activity in the mouse thymus cortex.