The most fibrogenic cytokine, TGF-β, is produced as a latent complex, in which active TGF-β is wrapped by latency associated protein (LAP), preventing TGF-β from binding its receptor on target cells. Therefore, active TGF-β must be released from latent complexto exert biological activities (TGF-β activation reaction). We elucidated that plasma kallikrein cleaves LAP between R⁵⁸ and L⁵⁹ residues and activates TGF-β in the pathogenesis of liver fibrosis. To detect degradated products of LAP (LAP-DPs), we produced two specific antibodies. One is the R⁵⁸ antibody detecting N-terminal side LAP-DPs terminating on R⁵⁸ residue (R⁵⁸ LAP-DPs), and another is L⁵⁹ antibody detecting C-terminal side LAP-DPs starting from L⁵⁹ residue (L⁵⁹ LAP-DPs). We established techniques to detect each LAP-DPs using the antibodies. The R⁵⁸ LAP-DPs remaining in tissues or cell surfaces through S-S bonded LTBP can be detected by immunostaining with the R⁵⁸ antibody, whereas the L⁵⁹ LAP-DPs released into blood can be measured by the ELISA using the L⁵⁹ antibody. These LAP-DPs can be a surrogate biomarker of TGF-ß activation and subsequent fibrogenesis.