In this work the conditions of labeling protein A with europium ions were studied and the conjugates obtained in this study were compared with traditional peroxidase conjugates currently used in immunochemistry. The conjugates of protein A with Eu3+ chelate were obtained with the use of cyclic dianhydride of diethylenetriaminepentaacetic acid (DADETPA). Conjugation methods with the use of DADETPA was shown to permit obtaining high-quality conjugates with europium chelates. Europium-labeled protein A ensured the sensitivity of the determination of adsorbed IgG at a level of 2 ng/ml and the dynamic analytical range within 3-1,000 ng/ml, which essentially exceeded similar characteristics of peroxidase conjugates with protein A. Europium-labeled protein A was used for the detection of antibodies to Francisella tularensis in the sera of humans immunized against tularemia. The sensitivity of this assay exceeded that of the enzyme immunoassay 10- to 40-fold. A conclusion was made on the possibility of using europium labelled protein A for the determination of specific antibodies to F.tularensis. This preparation may be useful in the determination of specific antibodies in low-immune sera.