Little is known about the role of conformation on the antigen processing by antigen presenting cells. Using a well-defined antigen containing two disulfide bridges, the synthetic C-terminal fragment 282-383 derived from Plasmodium falciparum circumsporozoite protein (PfCS 282-383), we show that the reduced form is presented in vitro more efficiently than its oxidized counterpart, inducing stronger CTL recognition. In addition, only the reduced form can be presented by the TAP independent T2 cell line. Thus, the reduced form is processed by TAP dependent and independent pathways.