Serodiagnostic tests are widely available for tick-borne diseases. We evalu
ated a cell-free antigen of the human granulocytic ehrlichiosis agent. Immu
nofluorescence assay (IFA) with this antigen is as efficient as with the MR
L kit and allows a one-step IFA with other cell-free antigens that is usefu
l when testing sera from patients bitten by ticks.