The species-related specificity of sera against 94-KD proteins isolated from culture filtrates of B. anthracis strains with different levels of virulence plasmids was studied to determine whether they might be used to identify the pathogen of anthrax. Sera against fractions 1 of culture filtrates of B. anthracis strains CTI (pXO1+ pXO2-), 81/1TR (pXO1- pXO2-), Davies (pXO1- pXO) separated by gel chromatography on Sephacryl S-300 were examined. In the gel immunodiffusion test with growing cultures, the sera exhibited non-identical antigens and differed in the presence of antibodies to antigens of related bacilli. The sera against fractions 1 of culture filtrates of toxin-producing and plasmidless strains displayed antigens produced only by B. anthracis strains into nutrient agar. Electroimmunotransblotting revealed that they contained antibodies mainly to 94-kD proteins and failed to react with B. cereus proteins with a molecular weight of 94 kD and with B. thuringiensis proteins with a molecular weight of 97 kD, which were extracted from autonomous cells. In the immunofluorescence test, immunoglobulins of sera against fractions 1 of culture filtrates of three strains stained autonomous cells and spores of 23 B. anthracis strains with different levels of virulence plasmids. In working dilutions, they did not react with antigens of 18 strains of related bacilli, which presents a possibility of using them for species identification of B. anthracis.