Coxiella burnetii cells in both phase I and II reveal in sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) similar protein profiles with only small differences. C. burnetii protein profile in SDS-PAGE depended on the method of purification of C. burnetii cells from chick embryo yolk sacs. Immune mouse sera against C. burnetii phase I cells recognized in phase I and II cell protein profiles mainly the 61 K and 29 K proteins by the immunoblot method. Hyperimmune mouse and rabbit sera against phase I and II cells reacted in different way with phase I and II cells. Sera against phase I cells recognized in both phase I and II profiles more protein bands than sera against phase II cells. Thus phase I LPS present in phase I cells exerted adjuvant effect on the antibody response in animals immunized with phase I cells.