In comparison to steers, bulls have produced 10% higher liveweight gain, 14% higher carcase gain and 20% more lean in the carcase. Immunological castration, which is reversible may reduce aggressive behaviour and maintain the performance advantages of bulls. Active immunization against luteinising hormone releasing hormone (LHRH) reduces testosterone production and therefore aggression in bulls. However, a controlled, predictable response to vaccination has not been achieved. Consistent immunization may be possible using a monoclonal antibody against LHRH. An anti-LHRH monoclonal antibody was developed and studies were carried out to determine the binding characteristics of the antibody and the half-life when administered to 6 post-pubertal bulls. The ability of the antibody to neutralize the bioactivity of LHRH was investigated in bulls and mice.