Sera from 65 persons with clinical brucellosis were employed in a comparison of standard and rapid serological tests. The results obtained with the Rose Bengal test correlated very well with those of the standard tube agglutination test, whereas results with the rapid plate agglutination test and the Coombs (antiglobulin) test were inferior. Absorption of patients' sera with specific anti-human immunoglobulin sera showed that IgM was active in the Rose Bengal test but not in the Coombs test, whereas IgG and IgA were active in both tests. In addition to the A & M antigen, which plays the most important role in the agglutination, Rose Bengal, and Coombs tests, other antigenic fractions of Brucella were examined in precipitation tests. A protein antigen reacted with 94% of the sera in counter-immunoelectrophoresis. On the basis of the results with both groups of sera, the Rose Bengal test and counter-immunoelectrophoresis appear to be the most promising methods for diagnosing clinical brucellosis. The tests differ qualitatively since different Brucella antigens are employed.