An enzyme-linked immunosorbent assay was developed to follow the bovine response, by immunoglobulin class and subclass, to defined smooth and rough lipopolysaccharides (LPS) of Brucella abortus. Binding to smooth LPS of immunoglobulin G1 (IgG1) and IgG2 in sera from Brucella-infected animals was significantly greater than binding in sera from normal uninfected animals. Competition or steric blocking among IgM, IgG1, and IgG2 for binding sites on smooth LPS was shown to occur. Binding of IgM to Brucella smooth LPS with sera from uninfected animals was elevated above the assay control levels, and attempts to eliminate this nonspecific IgM binding were not successful. The same levels of nonspecific IgM binding were also seen with Brucella rough LPS, Escherichia coli LPS, and Pseudomonas solanacearum LPS. Sera from some, but not all, Brucella-infected animals showed elevated binding of IgG1 and IgM to both E. coli LPS and Brucella rough LPS as well as to Brucella smooth LPS. This was interpreted as specific antibody. Cross-reactions between B. abortus smooth or rough LPS and E. coli LPS could not be shown by immunodiffusion.