The reactivity of sera from Anaplasma marginale-infected bovine with red blood cells and with purified anaplasma bodies was analyzed by electron immunomicroscopy. Red blood cells from non-infected and from anaplasma-infected cows and A. marginale bodies separated from parasitized erythrocytes, were incubated with control, pre-immune and immune sera followed by anti-bovine IgG-Peroxidase. Immune sera from cows infected with the venezuelan and Florida isolate reacted with red blood cell membranes from normal and infected bovines, while sera from non-infected cows did not. The immune sera also recognized epitopes localized on the cell wall, membrane and on unidentified intracellular structures of the purified anaplasma bodies. Thus, we propose that A. marginale infection may cause structural and biochemical modifications of the plasma membrane of the bovine red blood cells during its intraerythrocytic cycle. This in turn could elicit an autoimmune type of response against its own cells that would stimulate erythrophagocitosis. The strong reactivity of the immune sera with the Anaplasma bodies suggests that the bovine immune system also recognizes epitopes located on the parasite.