The anti-19P2 ligand monoclonal antibodies of the invention (in particular P2L-1Ca) have very high binding ability and can neutralize the arachidonic acid metabolite releasing activity of the 19P2 ligand. Therefore, they can be used, among others, as diagnostic, prophylactic and/or therapeutic agents for various diseases caused by some or other abnormality in the pituitary function modulating activity (e.g. prolactin secretion promoting activity), central nervous system modulating activity and pancreatic function modulating activity, among others, supposedly possessed by the 19P2 ligand.The immunoassay method using the monoclonal antibodies of the invention by the sandwich technique (in particular the sandwich technique using the monoclonal antibody and an antibody recognizing an intermediate portion of the 19P2 ligand) can assay the 19P2 ligand or a derivative thereof specifically and with high sensitivity. This assay method can be used in elucidating the physiological functions of the 19P2 ligand or a derivative thereof.