The authors developed monoclonal antibodies to Coxiella burnetti, the agent of Q fever. The selected monoclonal antibody, Cox1D8, did not cross-react with other bacteria and was used for early detection of C burnetti in shell vial cell cultures and for staining C burnetii in paraffin embedded tissues. Formalin or Bouin fixation did not alter the reactivity of the antigen with the antibody. This monoclonal antibody could be useful in the pathologic diagnosis of Q fever hepatitis and endocarditis.