The first demonstration of immune serum-mediated neutralization of Anaplasma marginale and identification of the involved surface antigens is presented. Anaplasma marginale initial bodies were purified from parasitized erythrocytes by using ultrasonic disruption and differential centrifugation. The initial bodies were morphologically intact, as determined by electron microscopy, were not agglutinated by anti-bovine erythrocyte sera, and were infective. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated minimal erythrocytic contamination in the purified initial bodies. Rabbit antisera against initial bodies completely neutralized the infectivity of purified initial bodies for splenectomized calves, whereas normal rabbit serum did not alter infectivity. Nine initial body proteins were surface radioiodinated by using lactoperoxidase, and five of these were precipitated by the neutralizing antibody (105,000, 86,000, 61,000, 36,000, and 31,000 apparent m.w.). The results demonstrate that initial body determinants capable of inducing neutralizing antibody are present. The identification of surface radiolabeled proteins immunoprecipitated by the neutralizing antibody provides candidates to be tested as protective immunogens in cattle.