cDNAs coding for the heavy and light chains of a bovine anti-testosterone IgG1 monoclonal antibody have been cloned and sequenced. These cDNAs are the first to be reported for functionally rearranged bovine immunoglobulin genes. Testosterone binding by the antibody encoded by the cDNAs has been verified by expression of the cDNAs in COS-1 cells and detection of anti-testosterone antibodies in transfected cell media using an ELISA specific for bovine anti-testosterone IgG. The derived protein sequence of the variable domains have suggested a possible binding model for the interaction between the antibody and testosterone. The derived protein sequence of the constant domains has been used to identify residues which could be involved in the selective transport of bovine IgG1 from blood plasma into colostrum at the time of parturition.