A myeloma cell was fused with the spleen cell from BALB/c mouse immunized against the purified PAS-4 to produce PAS-4 monoclonal antibody. Clones of hybridomas were screened for PAS-4 antibody using ELISA and immunoblotting. Hybridoma cells producing PAS-4 monoclonal antibody (KAS4) were injected intraperitoneally into BALB/c mouse for mass production of the monoclonal antibody, and 20 ㎎/mouse of the antibody were abtained. PAS-4 after hydrolysis with TFMS or N-glycanase was still strongly recognized by KAS4, indicating that the epitope of PAS-4 may be peptide. Protein A affinity chromatogaphy and SDS-PAGE analysis showed that KAS4 was immunoglobulin G (IgGI) class. Using KAS4 monoclonal antibody PAS-4 concentrations in milk were monitored by ELISA. PAS-4 concentration in milk at onset of parturition was 3 times higher than that in normal milk. However, the PAS-4 concentration was rapidly decreased at day 4 postparturition and then reached to the level observed in normal milk at day 7 post-parturition.