The mature-parasite-infected erythrocyte surface antigen (MESA) of Plasmodium falciparum is an antigenically variable, high molecular weight protein of trophozoites and schizonts that is located at the erythrocyte surface membrane. It is first synthesized at the late ring stage and continues to be synthesized until late schizogony. MESA cannot be detected on the external surface of erythrocytes infected by trophozoites and early schizonts but is located at the internal surface in association with the erythrocyte membrane skeleton. The degree of association with the membrane skeleton varies among parasite lines, being greater in knobby parasite lines. MESA is phosphorylated and is present in a similar location to another phosphoprotein, the ring-infected erythrocyte surface antigen (RESA). However, it differs from RESA in being detected at a later stage of asexual development of the parasite.