Both spleen and appendix of the rabbit contain two types of accessory cells that regulate the response of T-cells to concanavalin A. These accessory cells are found among non-T, non-B adherent cells and among B-cells. There is, so far, no evidence for a sequential interaction of these two cell types. In addition, there is an inhibitory adherent cell type in the appendix that interferes with the B-cell-regulated proliferative T-cell response.