Studies are described in which hybridoma technology is used to produce a variety of reagents for the characterization and manipulation of the bovine humoral immune system. Selected members of a set of murine monoclonal antibodies (MAb) specific for each of four major isotypes of bovine Ig constant regions, one specific for anti-bovine Ig constant regions as well as one specific for anti-bovine light chains are discussed. Interspecific fusion of bovine lymphocytes with the established mouse cell line, SP2/0 was used to produce a collection of stable hybridomas among which were found secretors of bovine IgG1, IgG2, IgM, IgA and bovine light chain. Interspecific fusion of SP2/0 with lymphocytes from a multiparous Holstein four days post immunization with Streptococcus agalactiae yielded MAb with specificity for the immunizing antigen. One of these hybridomas, LHRB 19.17, which displayed a particularly stable secretory phenotype, was used as an immunogen for the production of a library of murine monoclonal anti-idiotype antibodies. Competitive antigen binding analysis showed that 15 of the 24 anti-LHRB 19.17 idiotype antibodies isolated blocked the binding of the idiotype to its nominal antigen and so were candidates for evaluation as antigen mimics. Some of the ways in which monoclonal anti-idiotypes in particular, and monoclonal in general, might be of use in problems of animal disease are discussed.