Tick-borne Neoehrlichia mikurensis is the cause of neoehrlichiosis, an infectious disease that features fever and vascular events. Compromised B-cell immunity is a risk factor for severe neoehrlichiosis, indicating the importance of antibodies in host defense. The development of serological assays has been hampered by the difficulty of culturing these intracellular bacteria. Here we present the first serological test for N. mi...