Non-agglutinating Brucella antibodies in fifteen cases of chronic brucellosis were found to be immunoglobulin classes G and A. The use of specific antiglobulin reagents in the indirect antiglobulin test allows an easy analysis. Direct agglutination by IgM but not by IgG Brucella antibody is analogous to the situation that occurs in the Rh-D antigen—antibody system.