Immunization with Anaplasma marginale outer membranes induced immunity against clinical disease which correlated with antibody titer to outer membrane proteins, including a 19-kDa protein (N. Tebele, T. C. McGuire, and G. H. Palmer, Infect. Immun. 59:3199-3204, 1991). This 19-kDa protein, designated major surface protein 5 (MSP-5), was encoded by a single-copy 633-bp gene. The molecular mass of MSP-5, defined in immunoblots by binding to monoclonal antibody ANAF16C1, was conserved among all recognized species of Anaplasma: A. marginake, A. centrale, and A. ovis. Recombinant MSP-5, which absorbed the antibody reactivity of bovine immune serum to native MSP-5, was recognized by anti-A. marginale and anti-A. centrale immune sera in a competitive inhibition assay with monoclonal antibody ANAF16C1. The presence of antibody to the epitope defined by monoclonal antibody ANAF16C1 in all postinfection sera tested indicates that this epitope is a potential diagnostic antigen for use in identifying persistently infected cattle. Anaplasmosis is an important vector-borne hemoparasitic rickettsial disease of ruminant livestock in tropical and subtropical regions of the world. The disease, caused by Anaplasma marginale (24, 37) and Anaplasma centrale (38) in cattle and Anaplasma ovis in sheep (8, 18), is characterized by anemia, weight loss, abortion, and death (1). Survivors are lifelong carriers of the rickettsia (35). A strategy for the development of safe and efficacious vaccines for rickettsial diseases, including anaplasmosis, is to identify antigens capable of inducing protective immunity and to produce recombinant replicas of those antigens for use as subunit immunogens. The use of this strategy for anaplasmosis has resulted in identification of two surface proteins, major surface protein 1 (MSP-1) and MSP-2, that induce protective immunity against challenge (25, 28). Recently, significant protection against homologous challenge was reported after vaccination of cattle with an outer membrane fraction of the Norton (Zimbabwe) strain of A. marginale (36). A correlation between antibody titers to the A. marginale outer membrane proteins, which included a 19-kDa protein, designated MSP-5, and protective immunity against clinical disease was demonstrated (36). Monoclonal antibody (MAb) ANAF16C1, which defined the 19-kDa protein in the A. marginale outer membrane fraction from the Norton strain (36), was used to obtain and characterize the recombinant 19-kDa protein of this report.