Sera from infected cattle, cattle with persistent postvaccinal antibody and serologically positive noninfected cattle were fractionated into major immunoglobulin classes by DEAE-cellulose chromatography and by sucrose density gradient centrifugation. Each fraction was assayed for anti-Brucella activity by standard tube-agglutination test (STT), buffered tube-agglutination test (BTT), and complement-fixation test (CF). In the sera from experimentally infected cattle, anti-Brucella antibody was found by all tests in 6 DEAE fractions and in slow, fast, and sediment regions of the density gradient. Sera from cattle with persistent postvaccinal titers had STT activity in all 6 DEAE fractions, BTT activity in 5 fractions and CF activity in only 1 fraction. The STT and BTT activities were found in the slow and the sediment regions of the gradient; the CF activity was found only in the slow region. Sera from a chronically infected animal had STT and BTT activities in 2 DEAE fractions and CF activity in only 1. The STT, BTT, and CF activities were found in the slow and the sediment regions of the gradient. The principal antibody in sera from noninfected cattle was immunoglobulin M, which had all of the CF activity and most of the STT and BTT activities. Low levels of STT and BTT activities were found in 3 other DEAE fractions. Only STT and BTT activities were found in the fast and the sediment regions of the gradient.