Immunization of rabbits with concentrated Brucella abortus antigens for 10 weeks led to nearly total loss of serum bactericidal activity for this organism. β and γ globulin fractions of hyperimmune serum obtained by starch block electrophoresis had bactericidal activity. Sephadex G-200 gel filtration of the β globulin fraction obtained by starch block electrophoresis yielded a fraction consisting of IgM and another containing IgG and IgA; both had bacteriolytic activity. The latter subfraction was separated by column chromatography on DEAE-Sephadex into IgG and IgA (probably contaminated by 15% to 20% IgG). Bacteriolytic tests indicated that IgA had a prozone of inhibition of bacteriolysis, whereas IgM and IgG did not. Addition of excess IgA inhibited bacteriolysis produced by IgG and IgM. Bactericidal kinetics disclosed little kill by whole hyperimmune serum. Hyperimmune IgM and IgG killed Br, abortus rapidly but IgA fractions gave a lag of more than 3 min before any kill was measured. Addition of excess IgA markedly retarded the bactericidal kinetics of both IgG and IgM.