PURPOSE: To obtain the subject novel monoclonal antibody which exhibits a specific reactivity with 5-dihydrotestosterone and is useful for an assay system and so on to determine the 5-dihydrotestosterone level by assaying 5-α-reductase activity.

CONSTITUTION: An immunogen is injected into a mammal such as a mouse to give immunity. Subsequently the mouse is killed and cells are taken from its spleen and are fused with myeloma cells to obtain hybridomas which proliferate in vitro. Subsequently the hybridoma cell systems are screened to find one which produces an antibody against an antigen, and antibodies produced by each hybridoma system are screened to select one which has the highest affinity for 5-dihydrotestosterone as an immunogen and a low affinity for testosterone. This antibody selected is subjected to cloning and cultured to obtain the aimed monoclonal antibody which is specific for 5-dihydrotestosterone and is employed for an assay system and so on to determine the 5-dihydrotestosterone level by assaying 5-α-reductase activity.