SummaryFerritin‐labelling techniques at the ultrastructural level have shown that antiserum from August rats immune toP. bergheiinfection contains antibodies which bind to the surfaces of parasitized reticulocytes but not to uninfected cells. Two antibody specificities have been demonstrated by comparing antisera i absorbed with infected reticulocytes, ii absorbed with uninfected reticulocytes, and iii unabsorbed. Ferritin labelling was much increased with antiserum preabsorbed with uninfected reticulocytes, and also with heat‐inactivated serum, indicating a blocking effect on parasite‐specific antibody binding by cold‐reacting anti‐erythrocyte isoantibodies known to be present. Energy‐dependent aggregation, shedding and endocytosis of labelled material was observed at the surfaces of unfixed infected re