Bovine immunoglobulin preparations from normal serum and from sera containing antibodies against Brucella abortus interfered with the brucellacidal action of bovine serum, whereas unfractionated normal serum and antisera were not inhibitory. The inhibitory property of immunoglobulin appeared to be attributable to some anticomplementary property because it also interfered with serum-mediated hemolysis of antibody-coated erythrocytes. The supernatant phase obtained after ultracentrifugation of bovine anti-B abortus immunoglobulin did not inhibit brucellacidal activity of normal bovine serum. Results of this study indicate that bovine anti-B abortus immunoglobulin preparations contain microaggregates of protein that can inhibit the ability of bovine serum to kill B abortus. The most likely mechanism is nonspecific activation of complement by microaggregated immunoglobulin, which consumes complement and makes it unavailable for bactericidal activity.