Changes of lipopolysaccharide (LPS) of Coxiella burnetii strain Priscilla during chick embryo yolk sac passaging were observed by SDS-PAGE and immunoblot analysis. The course of LPS phase variation was similar to that found in other C. burnetti strains, i.e. a conversion of the phase I to the intermediate phase II after 10 passages. The intermediate phase II LPS of Priscilla strain was also detectable by immunoblot analysis using immune serum against Priscilla strain in the 30th passage.