Two preparations based on monoclonal antibodies to bacteria of the genus Brucella have been obtained. From the monoclonal preparations and globulins isolated from them erythrocyte diagnostica have been obtained with the use of amidol. Experiments on the cross indication of brucellae and other bacteria by means of these diagnostica and a similar preparation obtained from polyclonal serum have shown very high specificity of erythrocyte immunoreagents prepared from monoclonal antibodies.