The immune responsiveness of guinea pigs both to dinitrophenyl-poly-L-lysine (DNP-PLL) and to the lysine-rich random copolymer of L-glutamic acid and L-lysine (GL) is controlled by the 'poly-L-lysine gene'. We have previously demonstrated that accessory cells of responder strains can be made incapable of presenting DNP-PLL to responsive T cells in assays for proliferation, by in vitro exposure of the cells to GL before and during their exposure to DNP-PLL. We demonstrate here that the presence of anti-Ia antibody in the cultures does not interfere with the apparent competition of the two antigens for presentation by accessory cells. Furthermore, the two antigens do not compete for presentation when the accessory cells are exposed to them at 1 degree C, suggesting that endocytosis and/or other energy-requiring cellular events are necessary for the competition.